ath9k: rename ini_modes_rxgain_5g_xlna to ini_modes_rxgain_xlna

rename the variable as preparation for using the array with 2.4 GHz
band, etc.

Signed-off-by: Miaoqing Pan <miaoqing@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>