mwifiex: add CHAN_REGION_CFG command

This patch adds command preparation and response handling for
CHAN_REGION_CFG command. These changes are prerequisites for adding
custom regulatory domain support.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>