iwlwifi: correct frequency settings

After commit 59eb21a6
"cfg80211: Extend channel to frequency mapping for 802.11j"
we use uninitialized sband->band when assign channel frequencies, what
results that 5GHz channels have erroneous (zero) center_freq value.

Patch fixes problem and simplifies code a bit.

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