iio: lsiio: fix error code handling error

commit acf50b35
"tools:iio:lsiio: add error handling"
introduced error handling of errors returned from
read_sysfs_string(), but with a simple if (retval),
missing the fact that these functions return a positive
value if the read was successful.

As a result lsiio regresses and does not show any
devices on my filesystem. Fix this by checking for
only negative error codes.

Cc: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>