b43: fix extpa_gain check for 2GHz

On the 2GHz and and on the 5GHZ band only the extpa_gain setting from
the 5GHz band was checked. this patch makes it check the property from
the correct band.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>