mwifiex: device dump support for usb interface

Firmware dump on usb interface is different with current
sdio/pcie chipset, which is based on register operation.

When firmware hang on usb interface, context dump will be
upload to host using 0x73 firmware debug event.

This patch store dump data from debug event and send to
userspace using device coredump API.

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