PCI: brcmstb: Make two changes in MDIO register fields

PCI: brcmstb: Make two changes in MDIO register fields

The hardware has been updated with two changes to the MDIO packet
format.

The CMD field used to be 12 bits and now is only 1 bit. This change
is backwards compatible because the field's starting bit position is
unchanged, and the only commands we've used have values 0 and 1.

The PORT field's width has been changed from 4 bits to 5 bits. When
written, the new bit is not contiguous with the other four. However,
this change is backwards compatible because the driver never used
anything other than 0 for the port field's value.

Thus, update the existing code to handle new changes to the hardware
in a backwards-compatible manner.

Signed-off-by: Jim Quinlan <james.quinlan@broadcom.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/20250214173944.47506-8-james.quinlan@broadcom.com
[kwilczynski: commit log]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>