8250_core: Fix unwanted TX chars write

commit b08c9c317e3f7764a91d522cd031639ba42b98cc upstream.

On transmit-hold-register empty, serial8250_tx_chars
should be called only if we don't use DMA.
DMA has its own tx cycle.

Signed-off-by: Loic Poulain <loic.poulain@intel.com>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>