USB: serial: ftdi_sio: fix line-status over-reporting

FTDI devices use a receive latency timer to periodically empty the
receive buffer and report modem and line status (also when the buffer is
empty).

When a break or error condition is detected the corresponding status
flags will be set on a packet with nonzero data payload and the flags
are not updated until the break is over or further characters are
received.

In order to avoid over-reporting break and error conditions, these flags
must therefore only be processed for packets with payload.

This specifically fixes the case where after an overrun, the error
condition is continuously reported and NULL-characters inserted until
further data is received.

Reported-by: Michael Walle <michael@walle.cc>
Fixes: 72fda3ca ("USB: serial: ftd_sio: implement sysrq handling on
break")
Fixes: 166ceb69 ("USB: ftdi_sio: clean up line-status handling")
Cc: stable <stable@vger.kernel.org>	# v2.6.35
Signed-off-by: Johan Hovold <johan@kernel.org>