ice: Add helper function for clearing VPGEN_VFRTRIG

Create a helper function for clearing VPGEN_VFRTRIG as this needs to be
done on reset to notify the VF that we are done resetting it. Also, it
needs to be done on SR-IOV initialization/creation in case it was left
in a bad state after SR-IOV tear down.

Signed-off-by: Brett Creeley <brett.creeley@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>