cfg80211: Fix sanity check on 5 GHz when processing country IE

This fixes two issues with the sanity check loop when processing
the country IE:

1. Do not use frequency for the current subband channel check,
   this was a big fat typo.
2. Apply the 5 GHz 4-channel steps when considering max channel
   on each subband as was done with a recent patch.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>