staging/rdma/hfi1: Use pointer instead of struct name

Use sizeof(*p) instead of sizeof(struct foo) to fix checkpatch check:
CHECK: Prefer alloc(sizeof(*p)...) over alloc(sizeof(struct foo)...)

Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Jubin John <jubin.john@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>