IB/hfi1: Cache registers during state change

When the LCB is going offline, inopportune port queries can cause
benign error messages to be logged.  To deal with this, cache the
registers just before setting the LCB to offline, allowing queries to
return without eliciting the error.

Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>