b43: fix extpa_gain check for 2GHz

[Kalle Valo]

> 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 at hauke-m.de>

Thanks, applied to wireless-drivers.git.

Kalle Valo