staging/rdma/hfi1: Fix for module parameter hdrq_entsize when it's 0

If driver is loaded with parameter hdrq_entsize=0, then
there's a NULL dereference when the driver gets unloaded.
This causes a kernel Oops and prevents the module  from
being unloaded. This patch fixes this issue by making sure
-EINVAL gets returned when hdrq_entsize=0.

Reviewed-by: Chegondi, Harish <harish.chegondi@intel.com>
Reviewed-by: Haralanov, Mitko <mitko.haralanov@intel.com>
Signed-off-by: Sebastian Sanchez <sebastian.sanchez@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>