serial: 8250: Clear dma tx_err unconditionally

No need to check non-zeroness first and then clear. Just set to zero
unconditionally.

Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/3b885e7f-1372-3aa9-febd-34566ba25e3d@linux.intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>