mac80211: validate cipher scheme PN length better

Currently, a cipher scheme can advertise an arbitrarily long
sequence counter, but mac80211 only supports up to 16 bytes
and the initial value from userspace will be truncated.

Fix two things:
 * don't allow the driver to register anything longer than
   the 16 bytes that mac80211 reserves space for
 * require userspace to specify a starting value with the
   correct length (or none at all)

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