Revert "serial: 8250_dma: don't bother DMA with small transfers"

This reverts commit 9119fba0cfeda6d415c9f068df66838a104b87cb.

This commit prevents from sending "big" file using Bluetooth.
When sending a lot of data quickly through the Bluetooth interface, and
after a variable amount of data sent, transfer fails with error:
kernel: [ 415.247453] Bluetooth: hci0 hardware error 0x00

Found on T100TA.

After reverting this commit, send works fine for any file size.

Signed-off-by: Frederic Danis <[e-mail]>
Fixes: 9119fba0cfed (serial: 8250_dma: don't bother DMA with small transfers)
Cc: [e-mail]
Reviewed-by: Heikki Krogerus <[e-mail]>
Acked-by: Andy Shevchenko <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/8250/8250_dma.c

···
 		return 0;
 
 	dma->tx_size = CIRC_CNT_TO_END(xmit->head, xmit->tail, UART_XMIT_SIZE);
-	if (dma->tx_size < p->port.fifosize) {
-		ret = -EINVAL;
-		goto err;
-	}
 
 	desc = dmaengine_prep_slave_single(dma->txchan,
 					   dma->tx_addr + xmit->tail,