[arbel] Stop firmware only once on shutdown

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/infiniband/arbel.c

 	arbel_cmd_close_hca ( arbel );
 	arbel_free_icm ( arbel );
 	arbel_stop_firmware ( arbel );
-	arbel_stop_firmware ( arbel );
 	free_dma ( arbel->mailbox_out, ARBEL_MBOX_SIZE );
 	free_dma ( arbel->mailbox_in, ARBEL_MBOX_SIZE );
 	for ( i = ( ARBEL_NUM_PORTS - 1 ) ; i >= 0 ; i-- )