IB/hfi1: Fix serdes loopback set-up

Change serdes mode setting to use MISC_CONFIG_BITS in
VERIFY_CAP_LOCAL_LINK_WIDTH register. This method of
setting up serdes loopback is universally compatible
across all firmware versions.

Reviewed-by: Jakub Byczkowski <jakub.byczkowski@intel.com>
Signed-off-by: Jan Sokolowski <jan.sokolowski@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>