staging/rdma/hfi1: Increase SDMA descriptor queue size

A value of 2048 increased PSM performance while not impacting verbs
performance.

Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Reviewed-by: Mitko Haralanov <mitko.haralanov@intel.com>
Signed-off-by: Ignacio Hernandez <ignacio.hernandez@intel.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>