mwifiex: don't wait for main_process in shutdown_drv

main_process is not expected to be running when shutdown_drv function
is called. currently we wait for main_process completion in the
function.

Actually the caller has already made sure main_process is completed by
performing below actions.
(1) disable interrupts in if_ops->disable_int.
(2) set adapter->surprise_removed = true, main_process wont be queued.
(3) mwifiex_terminate_workqueue(adapter), wait for workqueue to be
completed.

This patch removes redundant wait code and takes care of related
cleanup.

Signed-off-by: Xinming Hu <huxm@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>