iwlwifi: improve timing handling with dual-mode

In dual-mode, a number of scenarios need to be
considered, and the firmware can be very picky
about them. Adjust the timing (most importantly
the beacon interval) according to the different
modes.

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