net: stmmac: fixed the pbl setting with DT

This patch fixed the pbl(programmable burst length) setting
using DT. Even though the default pbl is 8, If there is no
pbl property in device tree file, pbl is set 0 and it causes
bandwidth degradation.

Signed-off-by: Byungho An <bh74.an@samsung.com>
Signed-off-by: David S. Miller <davem@davemloft.net>