iwlagn: don't use CCK rates for P2P interfaces

P2P interfaces must not use CCK rates, only OFDM
rates are allowed. To set this up, we need to set
up the broadcast station to start with 6M instead
of starting with 1M.

Since the interface type can change, also reset
the broadcast station when RXON changes.

This will affect beacons as well.

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