wifi: cfg80211: fix off-by-one in element defrag

If a fragment is the last element, it's erroneously not
accepted. Fix that.

Fixes: f837a653 ("wifi: cfg80211: add element defragmentation helper")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230827135854.adca9fbd3317.I6b2df45eb71513f3e48efd196ae3cddec362dc1c@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>