arch: um: kmsg_dump: Use console_is_usable

arch: um: kmsg_dump: Use console_is_usable

All consoles found on for_each_console are registered, meaning that all
of them have the CON_ENABLED flag set. Since NBCON was introduced it's
important to check if a given console also implements the NBCON callbacks.
The function console_is_usable does exactly that.

Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://patch.msgid.link/20251121-printk-cleanup-part2-v2-2-57b8b78647f4@suse.com
Signed-off-by: Petr Mladek <pmladek@suse.com>