iio: cm36651: Fix read/write integration time function.

This patch is fixed [read/write] integration time function.
cm36651 have integration time from 1 to 640 milliseconds.
But, print more then the thousand second. when call *_integration_time attribute.
Because read_integration_time function return IIO_VAL_INT.
read integration time function is changed return IIO_VAL_INT_PLUS_MICRO;
And then .write_raw_get_fmt callback function for parse a fixed-point number from a string.
Some description is revised milliseconds unit.

v2: cm36651_write_int_time function fixed as it was.

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