mlx4: remove unnecessary variables and arguments

mlx4_qp_attach/detach_common() don't use hash variable, move it to find_entry()
static find_entry() in mcg.c doesn't use steer argument

Signed-off-by: Eugenia Emantayev <eugenia@mellanox.co.il>
Reviewed-by: Yevgeny Petrilin <yevgenyp@mellanox.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>