LinuxKPI: 802.11: add further checks for deflink.supp_rates[]

When setting the per-band supp_rates bitfield check for mandatory
rates only. We cannot easily say at that point for 2Ghz whether
11g is supported so assume these days it is not pure-b.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 73cd1c5d44ead3b84cebcc0c24278429afa9e6ec)