MIPS: BCM47XX: Don't try guessing NVRAM size on MTD partition

When dealing with whole flash content (bcm47xx_nvram_init_from_mem) we
need to find NVRAM start trying various partition sizes (nvram_sizes).
This is not needed when using MTD as we have direct partition access.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Patchwork: https://patchwork.linux-mips.org/patch/9652/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>