iio: Do not accept multiple '.' in fixed point numbers

Currently when parsing a fix-point number we silently skip any additional '.'
found in the string.  E.g. '1.2.3.4' gets parsed as '1.234'. This patch
disallows this and returns an error if more than one '.' is encountered.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>