serial: sunsu: drop low-latency workaround

The sunsu driver has been carrying a workaround for the infamous
low_latency behaviour of tty_flip_buffer_push() by dropping and
reacquiring the port lock in the interrupt handler since 2004.

Since commit a9c3f68f3c ("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: "David S. Miller" <davem@davemloft.net>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20210421095509.3024-24-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/sunsu.c

@@ -466,12 +466,8 @@ static irqreturn_t sunsu_serial_interrupt(int irq, void *dev_id)
 		if (status & UART_LSR_THRE)
 			transmit_chars(up);
 
-		spin_unlock_irqrestore(&up->port.lock, flags);
-
 		tty_flip_buffer_push(&up->port.state->port);
 
-		spin_lock_irqsave(&up->port.lock, flags);
-
 	} while (!(serial_in(up, UART_IIR) & UART_IIR_NO_INT));
 
 	spin_unlock_irqrestore(&up->port.lock, flags);