iwlwifi: unify scan start checks

Rather than duplicating all the checks and even
in case of errors accepting the scan request
from mac80211, we can push the checks to the
caller and in all error cases reject the scan
request right away (rather than accepting and
then saying it was aborted).

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Wey-Yi W Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>