serial: xilinx_uartps: drop low-latency workaround

Commit c8dbdc842d ("serial: xuartps: Rewrite the interrupt handling
logic") reworked the driver interrupt processing but also, without
comment, added an unnecessary workaround for the infamous low_latency
behaviour of tty_flip_buffer_push() which had been removed years
before.

Specifically, 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: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20210421095509.3024-27-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/xilinx_uartps.c

@@ -301,9 +301,8 @@ static void cdns_uart_handle_rx(void *dev_id, unsigned int isrstatus)
 		tty_insert_flip_char(&port->state->port, data, status);
 		isrstatus = 0;
 	}
-	spin_unlock(&port->lock);
+
 	tty_flip_buffer_push(&port->state->port);
-	spin_lock(&port->lock);
 }
 
 /**