MIPS: BCM47xx: Add helper variable for storing NVRAM length
This simplifies code just a bit (also maybe makes it a bit more
intuitive?) and will allow us to stop storing header. Right now we copy
whole NVRAM including its header to the internal buffer. It is not
needed to store a header as we don't access all these details like CRC,
flags, etc. The next improvement that should follow is copying only the
real contents.