mwifiex: clean pcie ring only when device is present

commit 4f7ba432 upstream.

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

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>