iio: cm36651: Changed return value of read function

A return value of callback have been changed to IIO_VAL_INT.
If not IIO_VAL_INT, driver will print wrong value(*_read_int_time).

A follow up patch will deal with a related bug in the new event handling
code.

Signed-off-by: Beomho Seo <beomho.seo@samsung.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>