wifi: ath9k: fix LNA selection in ath_ant_try_scan()

In 'ath_ant_try_scan()', (most likely) the 2nd LNA's signal
strength should be used in comparison against RSSI when
selecting first LNA as the main one. Compile tested only.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://msgid.link/20231211172502.25202-1-dmantipov@yandex.ru