iwlagn: fix scan tx antenna setting on 5Ghz band

Looks that we do not set correctly antennas when scanning
on 5Ghz band and when bluetooth is enabled, because
priv->cfg->scan_tx_antennas[band] is only defined for
IEEE80211_BAND_2GHZ.

To fix we check band before limiting antennas to first one.
This allow to remove hard coded cfg->scan_tx_antennas[band].

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