net: dsa: mt7530: use p5_interface_select as data type for p5_intf_sel

Use the p5_interface_select enumeration as the data type for the
p5_intf_sel field. This ensures p5_intf_sel can only take the values
defined in the p5_interface_select enumeration.

Remove the explicit assignment of 0 to P5_DISABLED as the first enum item
is automatically assigned 0.

Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Acked-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20240122-for-netnext-mt7530-improvements-1-v3-2-042401f2b279@arinc9.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>