mwgc: fix battery voltage conversion

MultiWii types.h says:

typedef struct {
  uint8_t  vbat;               // battery voltage in 0.1V steps
  uint16_t intPowerMeterSum;
  uint16_t rssi;              // range: [0;1023]
  uint16_t amperage;
} analog_t;

Which is deciVolt instead of Volt.

Mavlink has:

  voltage_battery	uint16_t	Battery voltage, in millivolts (1 = 1 millivolt)

So multiply by 100 instead of 1000. Compile tested only.

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>