mac80211: don't always advertise remain-on-channel

Not all devices are really capable of implementing
remain-on-channel, even if it is implemented in SW,
as they can't necessarily deal with channel changes
while associated.

Remove the WIPHY_FLAG_HAS_REMAIN_ON_CHANNEL and add
it only if either the driver has remain_on_channel
implemented in the driver/device.

Also add it to all drivers that advertise P2P right
now since those definitely have to have it working.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>