eeprom: at24: remove struct at24_client

We use member client only to get a reference to the associated struct
device, via &client->dev. However we can get the same reference from
the associated regmap, via regmap_get_device(regmap).
Therefore struct at24_client can be removed and replaced with a regmap
pointer.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>