mac80211: use HWSIM_STA_MAGIC in hwsim_check_sta_magic() and hwsim_set_sta_magic().

This patch fixes a typo in mac80211_hwsim.c:
use HWSIM_STA_MAGIC in hwsim_check_sta_magic() and hwsim_set_sta_magic()
instead of HWSIM_VIF_MAGIC.

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>