net: bcmgenet: Refactor bcmgenet_set_features()

In preparation for unconditionally enabling TX and RX checksum
offloads, refactor bcmgenet_set_features() a bit such that
__netdev_update_features() during register_netdev() can make sure
that features are correctly programmed during network device
registration.

Since we can now be called during register_netdev() with clocks
gated, we need to temporarily turn them on/off in order to have a
successful register programming.

We also move the CRC forward setting read into
bcmgenet_set_features() since priv->crc_fwd_en matters while
turning on RX checksum offload, that way we are guaranteed they
are in sync in case we ever add support for NETIF_F_RXFCS at some
point in the future.

Signed-off-by: Doug Berger <opendmb@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>