serial: 8250_dma: call serial8250_tx_dma unconditionally

Since we have the same check inside the function we may drop it away in
__dma_tx_complete().

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>