mwifiex: update current config_band info in start_ap

It was observed that AP beacons would not reflect correct regulatory
information upon starting AP in A band. This was because of missing
AP config band update in set_channel of start_ap.

Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>