staging: iio: replace strict_strto*() with kstrto*()

The usage of strict_strto*() is not preferred, because
strict_strto*() is obsolete. Thus, kstrto*() should be
used.

Previously, there were only strict_strtol(), strict_strtoul(),
strict_strtoull(), and strict_strtoll(). Thus, when converting
to the variables, only long, unsigned long, unsigned long long,
and long long can be used.

However, kstrto*() provides various functions handling all types
of variables. Therefore, the types of variables can be changed
properly.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>