Staging: rtl8712: Use mod_timer

This patch introduces the use of API function mod_timer
instead of driver specific function as it is a more
efficient and standard way to update the expire field of
an active timer.

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