RDMA/cm: Add accessors for CM_REQ transport_type

Access the two fields through wrappers, like all other fields, to make it
clearer what is happening.

Link: https://lore.kernel.org/r/20200116170037.30109-2-jgg@ziepe.ca


Tested-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>