net: stmmac: Set FIFO sizes for ipq806x

Commit eaf4fac4 ("net: stmmac: Do not accept invalid MTU values")
started using the TX FIFO size to verify what counts as a valid MTU
request for the stmmac driver.  This is unset for the ipq806x variant.
Looking at older patches for this it seems the RX + TXs buffers can be
up to 8k, so set appropriately.

(I sent this as an RFC patch in June last year, but received no replies.
I've been running with this on my hardware (a MikroTik RB3011) since
then with larger MTUs to support both the internal qca8k switch and
VLANs with no problems. Without the patch it's impossible to set the
larger MTU required to support this.)

Signed-off-by: Jonathan McDowell <noodles@earth.li>
Signed-off-by: David S. Miller <davem@davemloft.net>