staging: rtl8192u: remove unused dm_gpio_change_rf_callback()

This patch removes dm_gpio_change_rf_callback() because it is not called
anywhere and it resides inside an #ifdef RTL8192E guard while RTL8192E
is not defined.

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