staging:iio:ade7854: Fix error handling on read/write

The original code does not correctly handle the error related to I2C
read and write. This patch fixes the error handling related to all
read/write functions for I2C.

Signed-off-by: John Syne <john3909@gmail.com>
Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Fixes: 8d97a587 ("staging: iio: meter: new driver for ADE7754 devices")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>