staging: rtl8192u: fix incorrect type in assignment in ieee80211_tx.c

This patch fixes the following sparse warning:
ieee80211_tx.c:174:36: incorrect type in assignment (different base types)
ieee80211_tx.c:174:36: expected unsigned short [unsigned] [short] [usertype] <noident>
ieee80211_tx.c:174:36: got restricted __be16 [usertype] <noident>

by adding left side cast to __be16.

Signed-off-by: Martin Karamihov <martinowar@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>