staging: rtl8192u: remove dm_check_rfctrl_gpio()

This patch removes dm_check_rfctrl_gpio() because it does nothing when
RTL8192U is defined.

Signed-off-by: Xenia Ragiadakou <burzalodowa@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>