net: dsa: don't warn in dsa_port_set_state_now() when driver doesn't support it

ds->ops->port_stp_state_set() is, like most DSA methods, optional, and
if absent, the port is supposed to remain in the forwarding state (as
standalone). Such is the case with the mv88e6060 driver, which does not
offload the bridge layer. DSA warns that the STP state can't be changed
to FORWARDING as part of dsa_port_enable_rt(), when in fact it should not.

The error message is also not up to modern standards, so take the
opportunity to make it more descriptive.

Fixes: fd364541

 ("net: dsa: change scope of STP state setter")
Reported-by: Sergei Antonov <saproj@gmail.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Sergei Antonov <saproj@gmail.com>
Link: https://lore.kernel.org/r/20220816201445.1809483-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>