iio: cm32181: Change cm32181 ambient light sensor driver

Integration time of cm32181 is guessed about milliseconds.
But cm32181_read_als_it function return IIO_VAL_INT.
So fixed to return IIO_VAL_INT_PLUS_MICRO.
Next, add .write_raw_get_fmt callback function for call iio_str_to_fixpoint.

v2: cm32181_write_als_id function fixed as it was.

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