wifi: ath9k: use u32 for txgain indexes

Since both 'ar9550_hw_get_modes_txgain_index()' and
'ar9561_hw_get_modes_txgain_index()' never returns
negative values, prefer 'u32' over 'int' and adjust
'ar9003_hw_process_ini()' accordingly.

Suggested-by: Jeff Johnson <quic_jjohnson@quicinc.com>
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/20230823182401.196270-2-dmantipov@yandex.ru