brcmfmac: rename variable prec to more appropriate name, ie. fifo

The term prec (precedence) is different from the fifo number. Rename
use of prec with fifo to be consistent and clear.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>