iio: light: tcs3472: fix ATIME register write

The integration time is controlled by the ATIME register only.  However,
this register is written by i2c_smbus_write_word_data() in write_raw().

We actually don't need to write a subsequent register.  So just use
i2c_smbus_write_byte_data() instead.

Cc: Peter Meerwald <pmeerw@pmeerw.net>
Cc: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Acked-by: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>