USB: serial: ftdi_sio: fix extreme low-latency setting

Since commit 557aaa7f ("ft232: support the ASYNC_LOW_LATENCY
flag") the FTDI driver has been using a receive latency-timer value of
1 ms instead of the device default of 16 ms.

The latency timer is used to periodically empty a non-full receive
buffer, but a status header is always sent when the timer expires
including when the buffer is empty. This means that a two-byte bulk
message is received every millisecond also for an otherwise idle port as
long as it is open.

Let's restore the pre-2009 behaviour which reduces the rate of the
status messages to 1/16th (e.g. interrupt frequency drops from 1 kHz to
62.5 Hz) by not setting ASYNC_LOW_LATENCY by default.

Anyone willing to pay the price for the minimum-latency behaviour should
set the flag explicitly instead using the TIOCSSERIAL ioctl or a tool
such as setserial (e.g. setserial /dev/ttyUSB0 low_latency).

Note that since commit 0cbd81a9 ("USB: ftdi_sio: remove
tty->low_latency") the ASYNC_LOW_LATENCY flag has no other effects but
to set a minimal latency timer.
Reported-by: NAntoine Aubert <a.aubert@overkiz.com>
Fixes: 557aaa7f ("ft232: support the ASYNC_LOW_LATENCY flag")
Cc: stable@vger.kernel.org # v2.6.31: e3e574adSigned-off-by: NJohan Hovold <johan@kernel.org>

drivers/usb/serial/ftdi_sio.c

@@ -1805,8 +1805,6 @@ static int ftdi_sio_port_probe(struct usb_serial_port *port)
 
 	mutex_init(&priv->cfg_lock);
 
-	priv->flags = ASYNC_LOW_LATENCY;
-
 	if (quirk && quirk->port_probe)
 		quirk->port_probe(priv);