IB/hfi1: Remove blind constants from 16B update

These values were introduced as part of the 16B code to
account for the varying size of the LRH between the differing
packet formats.

Replace the blind constants with defines based on FIELD_SIZEOF()
calls.

Fixes: 5b6cabb0 ("IB/hfi1: Add 16B RC/UC support")
Reviewed-by: Don Hiatt <don.hiatt@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>