mwifiex: add region code information in debugfs

region code is an EEPROM setting received from firmware. Let's display
this in debugfs along with other information.

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