Change 119f92b already tried to fix it, but
forgets the 'direct' usecase.

The 'backend-storage-type' node is optional. Its default depends on the
capability of the used backend memory, which means "circular" or NULL.
The latter defaults to 'direct' in the routines.
If it is NULL, the devicetree fixup routine skips exporting a
'backend-storage-type' node to the kernel's devicetree.

But currently if the 'backend-storage-type' node is explicitly given as
'direct', it will be skipped silently and set to NULL instead. In this
case the user of the 'barebox-state' tool then ends up with the warning:

"No backend-storage-type found, using default"

which is annoying, because it was given.

Storing the given value will still use a NULL if the
'backend-storage-type' node isn't defined, but stores everything else if
it is defined. Then the 'backend-storage-type' node is present in the
kernel's devicetree as well.

Signed-off-by: Juergen Borleis <jbe@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>