mwifiex: add PCIe function level reset support

This patch implements pre and post FLR handlers to support PCIe FLR
functionality. Software cleanup is performed in pre-FLR whereas
firmware is downloaded and software is re-initialised in
post-FLR handler.

Following command triggers FLR.
echo "1" > /sys/bus/pci/devices/$NUMBER/reset

This feature can be used as a recovery mechanism when firmware gets
hang.

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