cfg80211: make handle_band() and handle_channel() wiphy specific

This allows us to make more wiphy specific judgements when
handling the channels later on.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>