iio_channel_convert(): Mask upper bits

The value for bits in the raw word above the number of real data bits is
undefined and may contain garbage. We need to mask those bytes out when
converting the value to make sure that those upper bits do not leak.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
get_short_attr_name(): Handle channels with modifiers

For channels with modifiers we need to strip the first three instead of the
first two elements to get the short name.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>