mwifiex: add debugfs file to read chip information

This patch add 'verext' debugfs item, which can be used to
get detailed chip specific information from our firmware.

Examples:
echo "1" > /sys/kernel/debug/mwifiex/mlan0/verext
cat /sys/kernel/debug/mwifiex/mlan0/verext

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