mac80211_hwsim: fix wmediumd_pid

Fix a few minor issues with wmediumd_pid:
a) make static
b) use u32 to match the snd_pid type
c) use ACCESS_ONCE since we don't lock it
d) don't explicitly initialize to 0

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>