iwlagn: send RXON timing before associating

The PAN functionality requires us to send the
timing including a valid DTIM period to the
microcode before associating, so request this
data from mac80211 and send it to the device.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>