mwgc: fix mavlink_msg_gps_raw_int_pack usage

mavlink_msg_gps_raw_int_pack is defined as:

static inline uint16_t mavlink_msg_gps_raw_int_pack(uint8_t system_id,
		uint8_t component_id, mavlink_message_t* msg, uint64_t time_usec,
		uint8_t fix_type, int32_t lat, int32_t lon, int32_t alt, uint16_t eph,
		uint16_t epv, uint16_t vel, uint16_t cog, uint8_t satellites_visible) {

Swap Course-Over-Ground and visible satelites

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
Merge pull request #7 from koenkooi/gps-struct

mwgc: fix mavlink_msg_gps_raw_int_pack usage