gps_inject_data: fixed integer overflow

- array length of data was increased without changing the data type of
the variable holding the length

Signed-off-by: RomanBapst <bapstroman@gmail.com>

msg/gps_inject_data.msg

@@ -2,7 +2,7 @@ uint64 timestamp		# time since system start (microseconds)
 
 uint32 device_id                # unique device ID for the sensor that does not change between power cycles
 
-uint8 len                       # length of data
+uint16 len                       # length of data
 uint8 flags                     # LSB: 1=fragmented
 uint8[300] data                 # data to write to GPS device (RTCM message)