staging: rtl8712: changed GetFrameSubType macro

Fixed a sparse warning.
GetFrameSubType and GetFrameType should cast to __le16. Furthermore
GetFramSubType should use le16_to_cpu instead of cpu_to_le16.

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>