dsa: lan9303: Fix mapping between DSA port number and PHY address

The 'phy' parameter supplied to lan9303_phy_read/_write was sometimes a
DSA port number and sometimes a PHY address. This isn't a problem as
long as they are equal.  But if the external phy_addr_sel_strap pin is
wired to 'high', the PHY addresses change from 0-1-2 to 1-2-3 (CPU,
slave0, slave1).  In this case, lan9303_phy_read/_write must translate
between DSA port numbers and the corresponding PHY address.

Fixes: a1292595

 ("net: dsa: add new DSA switch driver for the SMSC-LAN9303")
Signed-off-by: Christian Eggers <ceggers@arri.de>
Reviewed-by: Michal Kubiak <michal.kubiak@intel.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://patch.msgid.link/20240703145718.19951-1-ceggers@arri.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>