[MTD] bast-flash partitions fixup

Ensure the whole device is added if there are no partitions found on the
device, so that at least the flash can be read/written.

Replace some of the constants with their SZ_xxx counterparts

Signed-off-by: Ben Dooks <ben@simtec.co.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>