powerpc/powernv: Handle unknown OPAL errors in opal_nvram_write()

opal_nvram_write currently just assumes success if it encounters an
error other than OPAL_BUSY or OPAL_BUSY_EVENT. Have it return -EIO
on other errors instead.

Fixes: 628daa8d

 ("powerpc/powernv: Add RTC and NVRAM support plus RTAS fallbacks")
Cc: stable@vger.kernel.org # v3.2+
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Vasant Hegde <hegdevasant@linux.vnet.ibm.com>
Acked-by: Stewart Smith <stewart@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>