IB/rdmavt: Clean up distinction between port number and index

IB core uses 1 relative indexing for ports. All of our data structures
use 0 based indexing. Add an inline function that we can use whenever we
need to validate a legal value and try to convert a port number to a
port index at the entrance into rdmavt.

Try to follow the policy that when we are talking about a port from IB
core point of view we refer to it as a port number. When port is an
index into our arrays refer to it as a port index.

Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Reviewed-by: Harish Chegondi <harish.chegondi@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>