samples: nvs: fix flash device access

Get the correct flash device from the storage partition node.
The current code is relying on the invalid assumption that the
chosen zephyr,flash-controller node is the device which contains
the storage partition.

Reported-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>