tools: iio: make scale and offset files optional

Make scale and offset optional by adding -ENOENT check as not all
drivers implement them.

Signed-off-by: Joo Aun Saw <jasaw@dius.com.au>
Acked-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>