net: dsa: microchip: ensure each ksz_dev_ops has its own dsa_switch_ops

Currently we have a single dsa_switch_ops for 4 very distinct families
of switches, and many dsa_switch_ops methods are simply a dispatches
through ksz_dev_ops. That creates an avoidable level of indirection.

As a preparation for removing that indirection layer, create a separate
dsa_switch_ops structure wherever we have a ksz_dev_ops. These
structures are not yet used - ksz_switch_ops from ksz_common.c still is.
However, this reduces the noise from subsequent changes.

All new dsa_switch_ops are exact copies of ksz_switch_ops. But we need
to export function prototypes from ksz_common.c so that they are
callable from individual drivers.

Note that "individual drivers" are not actual separate kernel modules.
All of ksz8.c, ksz9477.c and lan937x_main.c are part of the same
ksz_switch.ko. Only the "register interface" drivers are different
modules (ksz9477_i2c.o for I2C, ksz_spi.o for SPI, ksz8863_smi.o for
MDIO). So we don't need to export any symbol.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Bastien Curutchet (Schneider Electric) <bastien.curutchet@bootlin.com>
Link: https://patch.msgid.link/20260505-clean-ksz-driver-v1-6-05d70fa42461@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>