net: stmmac: dwmac1000: fix define DMA_BUS_MODE_RPBL_MASK

DMA_BUS_MODE_RPBL_MASK is really 6 bits,
just like DMA_BUS_MODE_PBL_MASK.

Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Acked-by: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>