MIPS: BCM63XX: fix nvram checksum calculation

The current checksum calculation code does nothing except checking that
the first byte of nvram is 0 without actually checking the checksum.

Implement the correct checksum calculation by calculating the crc32 with
the checksum field set to 0.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Patchwork: http://patchwork.linux-mips.org/patch/4540