mac80211: correct fragmentation threshold check

The fragmentation threshold is defined to be including the
FCS, and the code that sets the TX_FRAGMENTED flag correctly
accounts for those four bytes. The code that verifies this
doesn't though, which could lead to spurious warnings and
frames being dropped although everything is ok. Correct the
code by accounting for the FCS.

(JWL -- The problem is described here:
 http://article.gmane.org/gmane.linux.kernel.wireless.general/32205 )

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