chardev/char: fix qemu_chr_is_busy() check

chardev/char: fix qemu_chr_is_busy() check

`mux_cnt` struct member never goes negative or decrements,
so mux chardev can be !busy only when there are no
frontends attached. This patch fixes the always-true
check.

Fixes: a4afa548fc6d ("char: move front end handlers in CharBackend")
Signed-off-by: Roman Penyaev <r.peniaev@gmail.com>
Cc: "Marc-André Lureau" <marcandre.lureau@redhat.com>
Cc: qemu-devel@nongnu.org
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-ID: <20241014152408.427700-2-r.peniaev@gmail.com>