mwifiex: add cfg80211 get_channel handler

This patch add cfg80211 get_channel handler for mwifiex.
The handler will be used to report current channel to upper
layer utility.

Signed-off-by: Xinming Hu <huxm@marvell.com>
Signed-off-by: Cathy Luo <cluo@marvell.com>
Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>