IB/{hfi1, qib, rdmavt}: Move ruc_loopback to rdmavt

This patch moves ruc_loopback() from hfi1 into rdmavt for code sharing
with the qib driver.

Reviewed-by: Brian Welty <brian.welty@intel.com>
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Venkata Sandeep Dhanalakota <venkata.s.dhanalakota@intel.com>
Signed-off-by: Harish Chegondi <harish.chegondi@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>