net: dsa: mt7530: rename p5_intf_sel and use only for MT7530 switch

The p5_intf_sel pointer is used to store the information of whether PHY
muxing is used or not. PHY muxing is a feature specific to port 5 of the
MT7530 switch. Do not use it for other switch models.

Rename the pointer to p5_mode to store the mode the port is being used in.
Rename the p5_interface_select enum to mt7530_p5_mode, the string
representation to mt7530_p5_mode_str, and the enum elements.

If PHY muxing is not detected, the default mode, GMAC5, will be used.

Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Signed-off-by: David S. Miller <davem@davemloft.net>