mwifiex: fix negative cmd_pending count

cmd_pending is increased in mwifiex_wait_queue_complete() and
decreased in mwifiex_complete_cmd() currently.
If there are two or more commands in the cmd_pending_q the main
worker thread will pick up next command from cmd_pending_q
automatically after finishing current command. As a result
mwifiex_wait_queue_complete() will not be called because
the command is alreay completed. This leads to a negative
number in cmd_pending count.

Fix it by increasing cmd_pending when a cmd is queued into
cmd_pending_q and decreasing when that cmd is recycled. For scan
commands we don't perform inc/dec operations until it's moved
from scan_pending_q to cmd_pending_q. This covers both
synchronous and asynchronous commands.

Reported-by: Daniel Drake <dsd@laptop.org>
Tested-by: Daniel Drake <dsd@laptop.org>
Tested-by: Marco Cesarano <marco@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>