slave_dev->name is only populated at this stage if it was specified
through a label in the device tree. However that is not mandatory.
When it isn't, the error message looks like this:

[    5.037057] fsl_enetc 0000:00:00.2 eth2: error -19 setting up slave PHY for eth%d
[    5.044672] fsl_enetc 0000:00:00.2 eth2: error -19 setting up slave PHY for eth%d
[    5.052275] fsl_enetc 0000:00:00.2 eth2: error -19 setting up slave PHY for eth%d
[    5.059877] fsl_enetc 0000:00:00.2 eth2: error -19 setting up slave PHY for eth%d

which is especially confusing since the error gets printed on behalf of
the DSA master (fsl_enetc in this case).

Printing an error message that contains a valid reference to the DSA
port's name is difficult at this point in the initialization stage, so
at least we should print some info that is more reliable, even if less
user-friendly. That may be the driver name and the hardware port index.

After this change, the error is printed as:

[    6.051587] mscc_felix 0000:00:00.5: error -19 setting up PHY for tree 0, switch 0, port 0
[    6.061192] mscc_felix 0000:00:00.5: error -19 setting up PHY for tree 0, switch 0, port 1
[    6.070765] mscc_felix 0000:00:00.5: error -19 setting up PHY for tree 0, switch 0, port 2
[    6.080324] mscc_felix 0000:00:00.5: error -19 setting up PHY for tree 0, switch 0, port 3
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>