staging: rtl8192u: Fix endianness warnings in ieee80211_rx.c

Fixes the endianness warning "restricted __le16 degrades to integer" by
converting __le16 to short before using it in bitmasks or in the
macro WLAN_FC_GET_STYPE.

Signed-off-by: Sebastian Haas <sehaas@deebas.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>