staging: rtl8712: get rid of flush_scheduled_work

This patch is preparation for following patch for error handling
refactoring.

flush_scheduled_work() takes (wq_completion)events lock and
it can lead to deadlock when r871xu_dev_remove() is called from workqueue.
To avoid deadlock sutiation we can change flush_scheduled_work() call to
flush_work() call for all possibly scheduled works in this driver,
since next patch adds device_release_driver() in case of fw load failure.

Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/6e028b4c457eeb7156c76c6ea3cdb3cb0207c7e1.1626895918.git.paskripkin@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>