storage/flash_map: Return -ENODEV from flash_area_open

The commit adds check, to flash_area_open, whether there is any
device driver attached and returns -ENODEV if there isn't any.
This works around a problem where flash_area_open succeeds but
consecutive read/write causes crash.
It is enough to check the condition, and return error, here as
the flash_area_open has to precede, and be checked for success,
any read/write operations.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>