mwifiex: debugfs: trigger device dump for usb interface

This patch extend device_dump debugfs function to make it
works for usb interface.

For command timeouts, USB firmware will automatically emit
firmware dump events, so we don't implement device_dump().

For user-initiated dumps, we trigger it by issue firmware
dump event command to firmware, as there is no command
response, do not start 10s timer.

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