ice: use ice_is_vf_trusted helper function

The ice_vc_cfg_promiscuous_mode_msg function directly checks
ICE_VIRTCHNL_VF_CAP_PRIVILEGE, instead of using the existing helper
function ice_is_vf_trusted. Switch this to use the helper function so
that all trusted checks are consistent. This aids in any potential
future refactor by ensuring consistent code.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Konrad Jankowski <konrad0.jankowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>