mwifiex: use fw_status register to wake up PCIe card

FW can be woken up even by accessing device registers; we need
not explicitily enable interrupts for doing this. Future PCIe
devices will not be woken up by writing to host registers.
This patch enables driver to wake up device by reading FW status
register.

Also devices with sleep cookie enabled need some more time before
proceeding with processing. Handle this by adding a delay loop.

Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>