iwlagn: split remain-on-channel

If we're associated and want to do P2P at the same
time, the scan for remain-on-channel is currently
limited to 80ms because of the way the device will
behave in that case. Instead of doing that, split
up the dwell times into little pieces. It will not
actually be a single big dwell time then, but will
be close enough. This improves robustness of P2P
in such scenarios.

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>