USB: ftdi_sio: fix status line change handling for TIOCMIWAIT and TIOCGICOUNT

Handling of TIOCMIWAIT was changed by commit 1d749f9a
 USB: ftdi_sio.c: Use ftdi async_icount structure for TIOCMIWAIT, as in other drivers

FTDI_STATUS_B0_MASK does not indicate the changed modem status lines,
it indicates the value of the current modem status lines. An xor is
still required to determine which lines have changed.

The count was only being incremented if the line was high. The only
reason TIOCMIWAIT still worked was because the status packet is
repeated every 1ms, so the count was always changing. The wakeup
itself still ran based on the status lines changing.

This change fixes handling of updates to the modem status lines and
allows multiple processes to use TIOCMIWAIT concurrently.

Tested with two processes waiting on different status lines being
toggled independently.

Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Cc: Uwe Bonnes <bon@elektron.ikp.physik.tu-darmstadt.de>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>