iwlwifi: mvm: fix extended dwell time

FW adds 10 msec for every dwell time in low band, so we need
to set 10 msec less.
Don't use extended dwell time when fragmented scan is needed
because FW adds 3 msec per probe and it can easily exceed
max out of channel time.

Fixes: c3e230b1 ("iwlwifi: mvm: add extended dwell time")
Signed-off-by: David Spinadel <david.spinadel@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>