staging/rdma/hfi1: use RCU_INIT_POINTER() when NULLing.

It is safe to use RCU_INIT_POINTER() to NULL a pointer, instead of
rcu_assign_pointer().
This results in slightly smaller/faster code.

Signed-off-by: Muhammad Falak R Wani <falakreyaz@gmail.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>