eeprom: at24: change nvmem stride to 1

Trying to read the MAC address from an eeprom that has an offset that
is not a multiple of 4 causes an error currently.

Fix it by changing the nvmem stride to 1.

Cc: stable@vger.kernel.org
Signed-off-by: David Lechner <david@lechnology.com>
[Bartosz: tweaked the commit message]
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>