powerpc/nvram: Fix function name in some errors messages.

'nvram_create_os_partition' should be 'nvram_create_partition'.
Use __func__ to have it right, as done elsewhere in this file.

Signed-off-by: Christophe Jaillet <christophe.jaillet@wanadoo.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>