iwlwifi: mvm: use defines instead of variables for shared dwell times

Most of the dwells are constant across different scan types.
Use defines instead of depending on scan type.
This is needed as preparation to having different scan type per
band.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>