ramips: mt7620: split gsw hw_init to mac_init, ephy_init

ramips: mt7620: split gsw hw_init to mac_init, ephy_init

in order for the option ephy-disable to work
without also needing ephy-base option,
we have to skip all the lines that write to mdio addresses that
assume those addresses do not have an external switch.

Otherwise, ephy ports will be disabled in hardware,
but register writes still happen as if they are enabled.

Split the functions so that other things are done first,
and ephy port setup can be skipped with a simple "return".

Tested on Engenius EPG600 (MT7620A ver:2 eco:3)
with QCA8337 external switch

Ref: cc6fd6fbb505 ("ramips: mt7620: add ephy-disable option to switch driver")
Signed-off-by: Michael Pratt <mcpratt@pm.me>