serial: meson: fix setting number of stop bits

The stop bit value as to be or'ed, so far this worked only just by chance
because AML_UART_STOP_BIN_1SB is 0.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>