net: enetc: create a common enetc_pf_to_port helper

commit 3fda749384174de8d6b9484eecc89428791e7256 from
git://source.codeaurora.org/external/qoriq/qoriq-components/linux



Even though ENETC interfaces are exposed as individual PCIe PFs with
their own driver instances, the ENETC is still fundamentally a
multi-port Ethernet controller, and some parts of the IP take a port
number (as can be seen in the PSFP implementation).

Create a common helper that can be used outside of the TSN code for
retrieving the ENETC port number based on the PF number. This is only
correct for LS1028A, the only Linux-capable instantiation of ENETC thus
far.

Note that ENETC port 3 is PF 6. The TSN code did not care about this
because ENETC port 3 does not support TSN, so the wrong mapping done by
enetc_get_port for PF 6 could have never been hit.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 87614b93)
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Meng Li <Meng.Li@windriver.com>