docs: i2c: slave-interface: return errno when handle I2C_SLAVE_WRITE_REQUESTED

In case backend is not ready, ie: fail to wakeup or initialization, on
the returning of the I2C_SLAVE_WRITE_REQUESTED event, bus driver should
aware the backend status and might auto sending NACK on the next
incoming bytes for I2C master to retry.

Signed-off-by: Quan Nguyen <quan@os.amperecomputing.com>
Links:https://lore.kernel.org/linux-arm-kernel/556fa9e1-c54b-8370-4de7-c2d3ec7d6906@os.amperecomputing.com/


[wsa: made some wording more precise]
Signed-off-by: Wolfram Sang <wsa@kernel.org>