eeprom: at24: readability tweak in at24_probe()

Use a helper variable for the size we want to allocate with
devm_kzalloc() and save an ugly line break.

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>