staging: rtl8712: changed u16 to __be16

Fixed sparse warning.
Just changed u16 to __be16 and typecasts.

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>