Staging: rtl8712: Eliminate use of _cancel_timer_ex

Use timer API function del_timer_sync instead of driver
specific function _cancel_timer_ex as besides deactivating
a timer, it ensures that the timer is stopped on all
CPUs before the driver exists. Also, definition of function
_cancel_timer_ex is removed as it is no longer needed after
this change.

This is done using Coccinelle and semantic patch used for
this is as follows:

@@ expression x; @@

- _cancel_timer_ex (&x);
+ del_timer_sync (&x);

Signed-off-by: Vaishali Thakkar <vthakkar1994@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>