net: dsa: b53: Fix calculating number of switch ports

[ Upstream commit cdb067d31c0fe4cce98b9d15f1f2ef525acaa094 ]

It isn't true that CPU port is always the last one. Switches BCM5301x
have 9 ports (port 6 being inactive) and they use port 5 as CPU by
default (depending on design some other may be CPU ports too).

A more reliable way of determining number of ports is to check for the
last set bit in the "enabled_ports" bitfield.

This fixes b53 internal state, it will allow providing accurate info to
the DSA and is required to fix BCM5301x support.

Fixes: 967dd82f ("net: dsa: b53: Add support for Broadcom RoboSwitch")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>