netdev: bfin_mac: use promiscuous flag for promiscuous mode

Rather than using the Receive All Frames (RAF) bit to enable promiscuous
mode, use the Promiscuous (PR) bit.  This lowers overhead at runtime as
we let the hardware process the packets that should actually be checked.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: David S. Miller <davem@davemloft.net>