staging: iio: max517: Fix iio_info changes

struct iio_info introduced a bug where the second channel of a MAX518 can't be
used. This commit fixes the typo (using max518 instead of the max517 struct).

Signed-off-by: Roland Stigge <stigge@antcom.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>