powerpc/nvram: Handle partition names >= 12 chars

The name field in the nvram_header can be < 12 chars, null-terminated,
or 12 chars without the null.  Handle this safely.

Signed-off-by: Jim Keniston <jkenisto@us.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>