iio: dac: ad5593r: Fix i2c read protocol requirements

iio: dac: ad5593r: Fix i2c read protocol requirements

For reliable operation across the full range of supported
interface rates, the AD5593R needs a STOP condition between
address write, and data read (like show in the datasheet Figure 40)
so in turn i2c_smbus_read_word_swapped cannot be used.

While at it, a simple helper was added to make the code simpler.

Fixes: 56ca9db862bf ("iio: dac: Add support for the AD5592R/AD5593R ADCs/DACs")
Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Cc: <Stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20220913073413.140475-2-nuno.sa@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>