net: dsa: mt7530: rework mt753[01]_setup

[ Upstream commit 6e19bc26 ]

Enumerate available cpu-ports instead of using hardcoded constant.

Suggested-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Stable-dep-of: 120a56b0 ("net: dsa: mt7530: fix network connectivity with multiple CPU ports")
Signed-off-by: Sasha Levin <sashal@kernel.org>