IB/core: Make gid_table_reserve_default() return void

gid_table_reserve_default() always returns zero. Make it return void and
simplify error checking.

rdma_port is already calculated, use that while calling
gid_table_reserve_default() instead of recalculating it.

Reviewed-by: Daniel Jurgens <danielj@mellanox.com>
Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>