powerpc/nvram: Shuffle code around in nvram_create_partition()

This error log stuff is really pseries specific. As a first step we move
the initialization of these variables to the caller of
nvram_create_partition(), which is also slightly reorganized so we
setup the free partition before we clear the new partition, so the
chance of an error during clear leaving us with invalid headers
is lessened.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>