gianfar: prevent fragmentation in DSA environments

The eTSEC register MRBLR defines the maximum space in
the RX buffers and is set to 1536 by gianfar. This
reasonably covers the common use case where the MTU
is kept at default 1500. In that case, the largest
Ethernet frame size of 1518 plus an optional
GMAC_FCB_LEN of 8, and an additional padding of 8
to handle FSL_GIANFAR_DEV_HAS_TIMER totals to 1534
and nicely fit within the chosen MRBLR.

Alas, if the eTSEC is attached to a DSA enabled switch,
the (E)DSA header extension (4 or 8 bytes) causes every
maximum sized frame to be fragmented by the hardware.

This patch increases the maximum RX buffer size by 8
and rounds up to the next multiple of 64, which the
hardware's defines as RX buffer granularity.

Signed-off-by: Zefir Kurtisi <zefir.kurtisi@neratec.com>
Signed-off-by: David S. Miller <davem@davemloft.net>