firmware: bcm47xx_nvram: inline code checking NVRAM size

Separated function was not improving code quality much (or at all).
Moreover it expected possible flash end address as argument and it was
returning NVRAM size.

The new code always operates on offsets which means less logic and less
calculations.

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