IB/hfi1: Ensure VL index is within bounds

Improve the safety of the code and ensure the array cannot be indexed
out of bounds when picking the CPU for a given SDMA engine.

Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Kaike Wan <kaike.wan@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>