Replace GPIO port idx identifier with zephyrs one

[ikerperezdelpalomar]

See if this values https://github.com/CodethinkLabs/bloodlight-firmware/blob/65f4308d6d197dc288723b762341b2c780d54633/firmware/src/led.c#L71 can be replaced by zephyr GPIO indexing method.

Currently port_idx just used in bl_led__get_pin_mask to get a pin mask per port which allows us to do one set for each port used, rather than setting each gpio pin individually, so it's not a priority.