iwlwifi: mvm: combine ssid_bitmap setting for regular scans

The only difference in the ssid_bitmap between LMAC and UMAC scans is
that in LMAC bits 1 to 20 are used, while in UMAC bits 0 to 19 are
used (*sigh*).  So we can combine the bitmap creation into a single
function and simply shift left if LMAC is used.

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