net: dsa: ksz8794: Fix the port numbering for this ETH switch IC

After the "dts: ksz8974: add support for ksz8863"
(SHA1: b0e4886d) commit the port numbering
for KSZ8794 was wrong.

This IC has 3 ETH ports (with linear offset), with port 4 being the
"master" with non linear offset (i.e. 0x50 instead of linear 0x40).

Such situation can be explained with similarity of this IC to ksz8795,
which has 4 LAN ports, and then the 0x40 offset is used.

Signed-off-by: Lukasz Majewski <lukma@denx.de>