firmware: bcm47xx_sprom: Fix -Wmissing-prototypes warnings

bcm47xx_sprom.h did not include a prototype for bcm47xx_fill_sprom()
therefore add one, and make sure we do include that header to fix
-Wmissing-prototypes warnings.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>