b43: Fix sparse warning in hw-tkip code

This fixes a sparse warning in the hardware-TKIP code:

drivers/net/wireless/b43/xmit.c:272:18: warning: incorrect type in assignment (different base types)
drivers/net/wireless/b43/xmit.c:272:18:    expected unsigned short [unsigned] [short] <noident>
drivers/net/wireless/b43/xmit.c:272:18:    got restricted unsigned short [usertype] <noident>

The code should work correctly with and without this patch applied.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>