tty: serial: move and cleanup vt8500_tx_empty()

Make vt8500_tx_empty() more readable by introducing a new local variable
and move the function before handle_tx(). That way we can reuse it in
there too.

Cc: <linux-arm-kernel@lists.infradead.org>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220920052049.20507-2-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>