tty: serial: meson: allow baud-rates lower than 9600

Devices like DCF77 receivers need the baud-rate to be as low as 50.

I have tested this on a Meson GXL device with uart_A.

Cc: Jiri Slaby <jslaby@suse.com>
Cc: Carlo Caione <carlo@caione.org>
Cc: Kevin Hilman <khilman@baylibre.com>
Cc: linux-amlogic@lists.infradead.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Thomas Rohloff <v10lator@myway.de>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>