mwifiex: access interrupt status only while holding lock

This patch fixes a bug for few instances where PCIe interrupt
status variable is accessed without holding spin lock.
This can result into missing interrupts.

Fix this by copying interrupt status to a local variable and
then using it for calling specific routine.

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>