iwlwifi: mvm: Refactor and fix max probe len computation

Move iwl_mvm_max_scan_ie_len function to scan.c and fix the
implementation to conform with the LMAC scan API.
Since the correct implementation would leave us with unacceptably
tiny probes, add a workaround which returns a larger value.
In current implementation it's possible that unified_scan_lmac and
unified_sched_scan_lmac would return -ENOBUFS.

Signed-off-by: Andrei Otcheretianski <andrei.otcheretianski@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>