Staging: rtl8712: Use del_timer

Use timer API function del_timer instead of driver
specific function _cancel_timer_ex as it is a standard
way for deactivating a timer.

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

@@ expression x; @@
- _cancel_timer_ex (&x);
+ del_timer (&x);

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