serial: amba-pl011: drop low-latency workaround

Commit ead76f329f ("ARM: 6763/1: pl011: add optional RX DMA to PL011
v2") added RX DMA support and also reproduced the workaround for the
infamous low_latency behaviour of tty_flip_buffer_push() by dropping and
reacquiring the port lock during receive processing.

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.

Note that the port lock is also dropped in the PIO path
(see pl011_rx_chars), but it is not clear whether this is still needed
by the DMA code added by the aforementioned commit.

Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Russell King <linux@armlinux.org.uk>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20210421095509.3024-6-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/amba-pl011.c

@@ -937,12 +937,10 @@ static void pl011_dma_rx_chars(struct uart_amba_port *uap,
 		fifotaken = pl011_fifo_to_tty(uap);
 	}
 
-	spin_unlock(&uap->port.lock);
 	dev_vdbg(uap->port.dev,
 		 "Took %d chars from DMA buffer and %d chars from the FIFO\n",
 		 dma_count, fifotaken);
 	tty_flip_buffer_push(port);
-	spin_lock(&uap->port.lock);
 }
 
 static void pl011_dma_rx_irq(struct uart_amba_port *uap)