net: bcmgenet: set RBUF_SKIP_FCS based on UniMAC CRC forwarding

When the UniMAC block is configured to forward the CRC as part of the
Ethernet frame (priv->crc_fwd_en, set by default), enabling the hardware
RX checksum block unveiled that the dma_rxchk_bit was never set in the
per-packet status bits (dma_flag in bcmgenet_desc_rx). This would make
the chksum_ok variable to be never set to 1, and the networking stack
would have to compute the packet checksums, which takes a substantial
amount of time.

In order for the RXCHK block to properly compute the packet checksum in
hardware, we also need to set the RBUF_SKIP_FCS bit accordingly.

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