USB: ch341: fix ignored TIOCMIWAIT mask

Make sure the TIOCMIWAIT mask is always honoured.

The CH341 interrupt status has a multiple-status changed flag which
indicates that multiple status changes has occurred since last interrupt
event. Unfortunately, if the final status is the same, there appears to
be no way to determine which signal(s) has changed (an even number of
times).

This means that the multiple-status flag should not be used in
TIOCMIWAIT as it leads to the signal mask argument being ignored (e.g.
TIOCMIWAIT could return if DSR changes twice, even though the user only
cares about carrier changes).

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>