mwifiex: clean pcie ring only when device is present

Write io memory to clean PCIe buffer only when PCIe device is
present else this results into crash because of invalid memory
access.

Cc: <stable@vger.kernel.org> # 3.9+
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>