Refactor driver by splitting the code into core and i2c.

Seemingly redundant read/write function parameters are required for
compatibility with the SPI driver.
Co-developed-by: Andreas Klinger <ak@it-klinger.de>
Signed-off-by: Andreas Klinger <ak@it-klinger.de>
Signed-off-by: Petre Rodan <petre.rodan@subdimension.ro>
Link: https://lore.kernel.org/r/20231229092445.30180-9-petre.rodan@subdimension.roSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>