Returning zero from the measurment function has the side effect of
corrupting the triggered buffer readings, better to use -EINVAL than
a zero measurement reading.

The INVALID status happens even it isn't out of range
sometimes roughly once every second or two. This can be from an
invalid second signal return path. Hence there are spurious zero
readings from the triggered buffer, and warning messages in the kernel
log.
Signed-off-by: Matt Ranostay <mranostay@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>