firmware: bcm47xx_nvram: rename finding function and its variables

1. Use "bcm47xx_" function name prefix for consistency
2. It takes flash start as argument so s/iobase/flash_start/
3. "off" was used for finding flash end so just call it "flash_size"

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>