mwifiex: remove global variable cmd_wait_q_required

There is a race condition while queuing synchronous command and
asynchronous command requested from different threads, because
the wait_q_enabled flag is set based on a global variable
cmd_wait_q_required.

The issue is fixed by removing this global variable and using a
unified function with an argument 'sync' passed into the
function.

Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>