serial: atmel_serial: drop low-latency workaround

Commit 1ecc26bd2789 ("atmel_serial: split the interrupt handler") worked
around the infamous low_latency behaviour of tty_flip_buffer_push() by
dropping and reacquiring the port lock in the tasklet callback.

Since commit a9c3f68f3cd8 ("tty: Fix low_latency BUG"),
tty_flip_buffer_push() always schedules a work item to push data to the
line discipline and there's no need to keep any low_latency hacks around.

Cc: Richard Genoud <[e-mail]>
Signed-off-by: Johan Hovold <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/atmel_serial.c

···
 			       1,
 			       DMA_FROM_DEVICE);
 
-	/*
-	 * Drop the lock here since it might end up calling
-	 * uart_start(), which takes the lock.
-	 */
-	spin_unlock(&port->lock);
 	tty_flip_buffer_push(tport);
-	spin_lock(&port->lock);
 
 	atmel_uart_writel(port, ATMEL_US_IER, ATMEL_US_TIMEOUT);
 }
···
 		uart_insert_char(port, status, ATMEL_US_OVRE, c.ch, flg);
 	}
 
-	/*
-	 * Drop the lock here since it might end up calling
-	 * uart_start(), which takes the lock.
-	 */
-	spin_unlock(&port->lock);
 	tty_flip_buffer_push(&port->state->port);
-	spin_lock(&port->lock);
 }
 
 static void atmel_release_rx_pdc(struct uart_port *port)
···
 		}
 	} while (head >= pdc->dma_size);
 
-	/*
-	 * Drop the lock here since it might end up calling
-	 * uart_start(), which takes the lock.
-	 */
-	spin_unlock(&port->lock);
 	tty_flip_buffer_push(tport);
-	spin_lock(&port->lock);
 
 	atmel_uart_writel(port, ATMEL_US_IER,
 			  ATMEL_US_ENDRX | ATMEL_US_TIMEOUT);