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