ARM: meson: serial: only disable tx irq on stop

Since disabling the transmit state machine still allows characters to
be transmitted when written to the UART write FIFO, simply disable the
transmit interrupt when the UART port is stopped.

This has not shown an improvement with the console issues when running
systemd, but seems like it should be done.

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