Increase AT timeout to 10s in AT_CellularSMS::send_sms

For some devices sending can be slow (as an example see SIM800, it can be up to 60s), command is being run properly but default timeout is returning an invalid error.
See https://www.elecrow.com/wiki/images/2/20/SIM800_Series_AT_Command_Manual_V1.09.pdf

connectivity/cellular/source/framework/AT/AT_CellularSMS.cpp

@@ -420,6 +420,7 @@ nsapi_size_or_error_t AT_CellularSMS::send_sms(const char *phone_number, const c
     }
 
     _at.lock();
+    _at.set_at_timeout(10s);
 
     int write_size = 0;
 
@@ -437,6 +438,7 @@ nsapi_size_or_error_t AT_CellularSMS::send_sms(const char *phone_number, const c
                 // sending can be cancelled by giving <ESC> character (IRA 27).
                 _at.cmd_start(ESC);
                 _at.cmd_stop();
+                _at.restore_at_timeout();
                 _at.unlock();
                 return write_size;
             }
@@ -482,6 +484,7 @@ nsapi_size_or_error_t AT_CellularSMS::send_sms(const char *phone_number, const c
             pdu_str = create_pdu(phone_number, message + i * concatenated_sms_length, pdu_len,
                                  sms_count, i + 1, header_len);
             if (!pdu_str) {
+                _at.restore_at_timeout();
                 _at.unlock();
                 return NSAPI_ERROR_NO_MEMORY;
             }
@@ -509,6 +512,7 @@ nsapi_size_or_error_t AT_CellularSMS::send_sms(const char *phone_number, const c
                     // sending can be cancelled by giving <ESC> character (IRA 27).
                     _at.cmd_start(ESC);
                     _at.cmd_stop();
+                    _at.restore_at_timeout();
                     _at.unlock();
                     delete [] pdu_str;
                     return msg_write_len;
@@ -523,13 +527,15 @@ nsapi_size_or_error_t AT_CellularSMS::send_sms(const char *phone_number, const c
             delete [] pdu_str;
             remaining_len -= concatenated_sms_length;
             if (_at.get_last_error() != NSAPI_ERROR_OK) {
+                _at.restore_at_timeout();
                 return _at.unlock_return_error();
             }
         }
     }
 
     _sms_message_ref_number++;
     nsapi_error_t ret = _at.get_last_error();
+    _at.restore_at_timeout();
     _at.unlock();
 
     return (ret == NSAPI_ERROR_OK) ? msg_len : ret;