eeprom: at24: simplify the i2c functionality checking

Save one call and make code prettier by checking the i2c functionality
in the beginning of at24_probe(), saving the relevant values and
reusing them later.

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>