ARM: meson: serial: use meson_uart_tx_empty() to wait for empty

Use the meson_uart_tx_empty() instead of a direct read of the status
register. This is easier to read and will ensure the UART's transmit
state machine is idle when trying to update the baud rate.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Tested-by: Carlo Caione <carlo@endlessm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>