mlxsw: spectrum_flower: Fail in case user specifies multiple mirror actions

The ASIC can only mirror a packet to one port, but when user is trying
to set more than one mirror action, it doesn't fail.

Add a check if more than one mirror action was specified per rule and if so,
fail for not being supported.

Fixes: d0d13c18

 ("mlxsw: spectrum_acl: Add support for mirror action")
Signed-off-by: Danielle Ratson <danieller@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>