brcmutil: use define for boardrev string function

Introducing a define that the caller of brcmu_boardrev_str() can
use to allocate enough room for buffer passed to the function.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>