ARM: meson: serial: check for tx-irq enabled in irq code

Ensure that if the interrupt handler is entered then only try and do tx
work if the tx irq is enabled.

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>