vconsole-setup: downgrade log message when setting font fails on dummy console

Since commit 883eb9be, vconsole-setup might be
called again to operate on dummy console where font operations are not
supported but where it's still important to have the correct keymap set [0][1].

vconsole-setup is mainly called by udev but can also be run via a dependency of
an early service. Both cases might end up calling vconsole-setup on the dummy
console.

The first case can happen during early boot even on systems that use (instead
of the dummy console) a "simple" video console driver supporting font
operations (such as vgacon) until a more specific driver (such as i915) takes
the console over. While this is happening vgacon is deactivated and temporarly
replaced by the dummy console [2].

There are also other cases where systemd-vconsole-setup might be called on
dummy console especially during (very) early boot. Indeed
systemd-vconsole-setup.service might be pulled in by early interactive services
such as 'dracut-cmdline-ask.service` which is run before udev.

If that happens on platforms with no grapical HWs (such as embedded ARM) or
with dummy console initially installed until a driver takes over (like Xen and
xen-fbfront) then setting font will fail.

Therefore this patch downgrades the log message emitted when setting font fails
to LOG_DEBUG and when font operations is not implemented like it's the case for
the dummy console.

Fixes: #16406.

[0] https://github.com/systemd/systemd/issues/10826
[1] https://bugzilla.redhat.com/show_bug.cgi?id=1652473
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/gpu/vga/vgaarb.c?h=v5.7#n204