staging:iio:adc: Use kstrtol()/kstrtoul()

Replace deprecated strict_strtol()/strict_strtoul() with
kstrtol()/kstrtoul(). Add missing checks for conversion return codes.

Signed-off-by: Aida Mynzhasova <ai.c.c0der@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>