staging: rtl8192u: Fix warnings about endianness

drivers/staging/rtl8192u/ieee80211/ieee80211_rx.c:564:37: warning: incorrect type in assignment (different base types)
drivers/staging/rtl8192u/ieee80211/ieee80211_rx.c:564:37:    expected unsigned short [unsigned] [usertype] len
drivers/staging/rtl8192u/ieee80211/ieee80211_rx.c:564:37:    got restricted __be16 [usertype] <noident>

Signed-off-by: maomao xu <albert008.xu@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>