serial: amba-pl010: drop low-latency workaround

Commit 2389b272 ("[ARM] 4417/1: Serial: Fix AMBA drivers locking")
worked around the infamous low_latency behaviour of
tty_flip_buffer_push() by simply dropping and reacquiring the port lock
in the interrupt handler.

Since commit a9c3f68f ("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: Russell King <linux@armlinux.org.uk>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20210421095509.3024-5-johan@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/amba-pl010.c

@@ -159,9 +159,7 @@ static void pl010_rx_chars(struct uart_amba_port *uap)
 	ignore_char:
 		status = readb(uap->port.membase + UART01x_FR);
 	}
-	spin_unlock(&uap->port.lock);
 	tty_flip_buffer_push(&uap->port.state->port);
-	spin_lock(&uap->port.lock);
 }
 
 static void pl010_tx_chars(struct uart_amba_port *uap)