net: dpaa2: replace dpaa2_mac_is_type_fixed() with dpaa2_mac_is_type_phy()

dpaa2_mac_is_type_fixed() is a header with no implementation and no
callers, which is referenced from the documentation though. It can be
deleted.

On the other hand, it would be useful to reuse the code between
dpaa2_eth_is_type_phy() and dpaa2_switch_port_is_type_phy(). That common
code should be called dpaa2_mac_is_type_phy(), so let's create that.

The removal and the addition are merged into the same patch because,
in fact, is_type_phy() is the logical opposite of is_type_fixed().

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Tested-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>