bgmac: fix MAC soft-reset bit for corerev > 4

Only core revisions older than 4 use BGMAC_CMDCFG_SR_REV0. This mainly
fixes support for BCM4708A0KF SoCs with Ethernet core rev 5 (it means
only some devices as most of BCM4708A0KF-s got core rev 4).
This was tested for regressions on BCM47094 which doesn't seem to care
which bit gets used.

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