LinuxKPI: 802.11 ieee80211_sn_sub() fix

In ieee80211_sn_sub() we need to shift the mask before applying it.
This fixes the logic from 978f25e840fe4975f243e6077a764842f0e9d3eb.

Reported by: J.R. Oldroyd (fbsd opal.com)
MFC after: 3 days
Sponsored by: The FreeBSD Foundation