iwlwifi: scan: Create function to build scan cmd

Currently, the code to build scan cmd is duplicated in
iwl_mvm_reg_scan_start and iwl_mvm_sched_scan_start.

Create a function to build this command, and call the function instead.

Signed-off-by: Tova Mussai <tova.mussai@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>