eeprom: at24: provide and use at24_base_client_dev()

Use a helper function for accessing the device struct of the base
i2c client. This routine is named in a way that reflects its purpose
unlike the previously hand-coded dereferencing.

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Tested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>