IB/hfi1: Right size user_sdma sequence numbers and related variables

Hardware limits the maximum number of packets to u16 packets.

Match that size for all relevant sequence numbers in the user_sdma
engine.

Reviewed-by: Mike Marciniszyn <mike.marciniszyn@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: Jason Gunthorpe <jgg@mellanox.com>