samples: drivers: flash_shell: use DEVICE_DT_GET_OR_NULL

The optional flash device can be obtained at compile time using
DEVICE_DT_GET_OR_NULL. Init code has been adjusted so that flash_dev is
forced to NULL when device is not ready (user can later specify the
device manually).

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>