Revert "mtd: bcm47xxpart: improve probing of nvram partition"

This reverts commit be3781b7.

Some CFE bootloaders have NVRAM at offset 0x1000. With that patch we
were detecting such a bootloaders as a standard NVRAM partition.
Changing anything in this pseudo-NVRAM resulted in corrupted bootloader
and bricked device!

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>