iwlwifi: mvm: combine LMAC and UMAC preq generation

The probe request to be added to both LMAC and UMAC scan commands are
identical, so move the generation out of the LMAC/UMAC-specific code.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>