mtd: phram: fix memory leak

Commit 4f678a58 (mtd: fix memory leaks in phram_setup) missed two cases
where the memory allocated for name would be leaked. This commit frees
the memory when register_device() fails and on unregister_devices().

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>