MIPS: BCM47xx: Add helper variable for storing NVRAM length

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.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Cc: linux-mips@linux-mips.org
Cc: Arend van Spriel <arend@broadcom.com>
Cc: Hante Meuleman <meuleman@broadcom.com>
Patchwork: https://patchwork.linux-mips.org/patch/10535/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>