net80211: allow a single wlan crypto module to register for >1 cipher

This allows a single wlan crypto module to register for more than
one cipher. Without it, duplicate linkerset structs are initialised
for the actual module loading machinery itself.

I've tested this in my private tree with wlan_ccmp providing both
128 and 256 bit cipher support.

Differential Revision: https://reviews.freebsd.org/D44901
Reviewed by: bz, cc, cy
Approved by: bz, cc, cy