iwlwifi: mvm: combine scan size checks into a common function

Instead of repeating the same code in 4 different places, combine the
comparisons into a new function.  Additionally, this change fixes UMAC
scans where the RRM IEs were not taken into consideration when
calculating the IE length.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>