bgmac: reset & enable Ethernet core before using it

This fixes Ethernet on D-Link DIR-885L with BCM47094 SoC. Felix reported
similar fix was needed for his BCM4709 device (Buffalo WXR-1900DHP?).
I tested this for regressions on BCM4706, BCM4708A0 and BCM47081A0.

Cc: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>