net: dsa: mt7530: drop paranoid checks in .get_tag_protocol()

It is desirable to reduce the surface of DSA_TAG_PROTO_NONE as much as
we can, because we now have options for switches without hardware
support for DSA tagging, and the occurrence in the mt7530 driver is in
fact quite gratuitout and easy to remove. Since ds->ops->get_tag_protocol()
is only called for CPU ports, the checks for a CPU port in
mtk_get_tag_protocol() are redundant and can be removed.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: DENG Qingfang <dqfext@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>