mwifiex: add fw_dump debugfs file

This option be useful to dump firmware memory for debugging
purpose. Actual code to dump firmware momory for SDIO and PCIe
chipsets will be added later.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>