iwlwifi: always restrict scan dwell in P2P

Whenever the PAN (P2P) context is active, it
has timers in the uCode that prevent sleep,
so scanning can't be out of channel for more
than the beacon interval programmed into the
device.

Before this patch, a full scan including any
passive channels when P2P was active would
stall forever because it wouldn't find time
to execute the passive requests (for default
beacon intervals of 100 TU.)

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