eeprom: at24: call read/write functions via function pointers

The first step in simplifying the read and write functions is to call
them via function pointers stored in at24_data. When we eventually
split the routines into smaller ones (depending on whether they use
smbus or i2c operations) we'll simply assign them to said pointers
instead of checking the flags at runtime every time we read/write.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>