net: dsa: ksz8794: Fix the port numbering for DSA tag setting

After the commit "dts: ksz8974: change port number to start at zero."
(SHA1: aa621769)

the lan ports are numbered starting from zero (not as previously from
one), so code assigning tags shall take this into account.

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