mac80211: sequence number micro-optimisation

There's no need to mask the variable with 0xFFF0
since we ever only use it as a u16 and the lowest
four bits can't ever be non-zero. The compiler
cannot infer the latter, and therefore has to emit
code to do the masking.

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