serial: mpc52xx_uart: drop low-latency workaround

Commit fbe543b4 ("Fix a potential issue in mpc52xx uart driver")
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.
Signed-off-by: NJohan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20210421095509.3024-16-johan@kernel.orgSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/mpc52xx_uart.c

@@ -1421,9 +1421,7 @@ mpc52xx_uart_int_rx_chars(struct uart_port *port)
 		}
 	}
 
-	spin_unlock(&port->lock);
 	tty_flip_buffer_push(tport);
-	spin_lock(&port->lock);
 
 	return psc_ops->raw_rx_rdy(port);
 }