firmware: bcm47xx_nvram: _really_ correct size_t printf format

Commit feb4eb06 ("firmware: bcm47xx_nvram: Correct size_t printf
format") was wrong, and changed a printout of 'header.len' - which is an
u32 type - to use '%zu'.

It apparently did pattern matching on the other case, where it printed
out 'nvram_len', which is indeed of type 'size_t'.

Rather than undoing the change, this just makes it use the variable that
the change seemed to expect to be used.

Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Philippe Mathieu-Daudé <f4bug@amsat.org>
Cc: Paul Burton <paul.burton@mips.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>