wifi: ath9k: simplify ar9003_hw_process_ini()

Since commit 8896934c ("ath9k_hw: remove direct accesses to channel
mode flags") changes 'ar9550_hw_get_modes_txgain_index()' so it never
returns -EINVAL, and 'ar9561_hw_get_modes_txgain_index()' never returns
negative value too, an extra check in 'ar9003_hw_process_ini()' may be
dropped.

Found by Linux Verification Center (linuxtesting.org) with SVACE.
Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Reviewed-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20230823182401.196270-1-dmantipov@yandex.ru

drivers/net/wireless/ath/ath9k/ar9003_phy.c

@@ -925,9 +925,6 @@ static int ar9003_hw_process_ini(struct ath_hw *ah,
 			modes_txgain_index =
 				ar9561_hw_get_modes_txgain_index(ah, chan);
 
-		if (modes_txgain_index < 0)
-			return -EINVAL;
-
 		REG_WRITE_ARRAY(&ah->iniModesTxGain, modes_txgain_index,
 				regWrites);
 	} else {