net: dsa: autoload tag driver module on tagging protocol change

Issue a request_module() call when an attempt to change the tagging
protocol is made, either by sysfs or by device tree. In the case of
ocelot (the only driver for which the default and the alternative
tagging protocol are compiled as different modules), the user is now no
longer required to insert tag_ocelot_8021q.ko manually.

In the particular case of ocelot, this solves a problem where
tag_ocelot_8021q.ko is built as module, and this is present in the
device tree:

&mscc_felix_port4 {
	dsa-tag-protocol = "ocelot-8021q";
};

&mscc_felix_port5 {
	dsa-tag-protocol = "ocelot-8021q";
};

Because no one attempts to load the module into the kernel at boot time,
the switch driver will fail to probe (actually forever defer) until
someone manually inserts tag_ocelot_8021q.ko. This is now no longer
necessary and happens automatically.

Rename dsa_find_tagger_by_name() to denote the change in functionality:
there is now feature parity with dsa_tag_driver_get_by_id(), i.o.w. we
also load the module if it's missing.

Link: https://lore.kernel.org/lkml/20221027113248.420216-1-michael@walle.cc/
Suggested-by: Michael Walle <michael@walle.cc>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Michael Walle <michael@walle.cc> # on kontron-sl28 w/ ocelot_8021q
Tested-by: Michael Walle <michael@walle.cc>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>