eeprom: at24: split at24_eeprom_read() into specialized functions

Split at24_eeprom_read() into two smaller functions - one for the
i2c operations and one for the smbus extensions. Assign them in
at24_probe() depending on the bus capabilities.

Also: in order to avoid duplications move the comments related to
offset calculations above the at24_translate_offset() routine.

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