drivers: flash: Fix flash shell to work without dts chosen node

The flash shell can work with any flash driver instance, not just the
one chosen by zephyr,flash-controller. It's helpful for the flash shell
to use this instance by default, but not required. We can switch
instances at runtime with the "flash set_device" command.

Fix the flash shell so it can build when there isn't a chosen
zephyr,flash-controller available.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>