tools:iio:iio_utils: check amount of matches

fscanf() usually returns the number of input items successfully matched
and assigned, which can be fewer than provided (or even zero).
Add a check in iioutils_get_type() to make sure all items are matched.

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>