mwifiex: fix SDIO interrupt lost issue

601216e1 "mwifiex: process RX packets in SDIO IRQ thread directly"
introduced a command timeout issue which can be reproduced easily on
an AM33xx platform using a test application written by Daniel Mack:

https://gist.github.com/zonque/6579314



mwifiex_main_process() is called from both the SDIO handler and
the workqueue. In case an interrupt occurs right after the
int_status check, but before updating the mwifiex_processing flag,
this interrupt gets lost, resulting in a command timeout and
consequently a card reset.

Let main_proc_lock protect both int_status and mwifiex_processing
flag. This fixes the interrupt lost issue.

Cc: <stable@vger.kernel.org> # 3.7+
Reported-by: Sven Neumann <s.neumann@raumfeld.com>
Reported-by: Andreas Fenkart <andreas.fenkart@streamunlimited.com>
Tested-by: Daniel Mack <zonque@gmail.com>
Reviewed-by: Dylan Reid <dgreid@chromium.org>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: Paul Stewart <pstew@chromium.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>