rtl8723au: Fix smatch problems in hal/rtl8723a_mp.c

rtl8723au: Fix smatch problems in hal/rtl8723a_mp.c

Smatch lists the following:
  CHECK   /home/finger/rtl8723au/hal/rtl8723a_mp.c
/home/finger/rtl8723au/hal/rtl8723a_mp.c:538 Hal_SetAntennaPathPower() error: buffer overflow 'TxPowerLevel' 2 <= 2
/home/finger/rtl8723au/hal/rtl8723a_mp.c:581 Hal_SetTxPower() error: buffer overflow 'TxPowerLevel' 2 <= 2
/home/finger/rtl8723au/hal/rtl8723a_mp.c:599 Hal_SetTxAGCOffset() info: ignoring unreachable code.
/home/finger/rtl8723au/hal/rtl8723a_mp.c:599 Hal_SetTxAGCOffset() info: ignoring unreachable code.

The buffer overflow errors are again an off-by=one situation.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>