staging: rtl8712: used a better macro

Fixed a sparse warning.
Using be16_to_cpus() to avoid double assignment.

Signed-off-by: Jannik Becher <becher.jannik@gmail.com>
Tested-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>