IB/hfi1: Ignore link downgrade with 0 lanes

Versions of the 8051 firmware < 0.38 may report a link failure
as a link downgrade with a width of 0 followed by a link down
notification.  Ignore the zero width downgrade notification -
the driver should follow the link down path.

Reviewed-by: Easwar Hariharan <easwar.hariharan@intel.com>
Signed-off-by: Dean Luick <dean.luick@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>