IB/hfi1: Remove unnecessary initialization from tx request

The tx request is unnecessarily initialized in the hot
code path with memset(), however, there's no need to do
this as most fields are initialized later on. this
initialization shows to be costly in the profile.
Remove unnecessary initialization from tx request and make
sure all variables are initialized properly.

Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Sebastian Sanchez <sebastian.sanchez@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>