staging: rtl8712: Coding style: Make use of BIT macro

* Replace all occurences of (1<<x) by BIT(x) in order to get rid
  of checkpatch.pl "CHECK" output "Prefer using the BIT macro"

Signed-off-by: Michael Hornung <mhornung.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>