fm10k: fix Rx buffer size

fm10k: fix Rx buffer size

As RX buffer is aligned to 512B within mbuf, some bytes are reserved
for this purpose, and the worst case could be 511B. But SRR reg
assumes all buffers have the same size. In order to fill the gap,
we'll have to consider the worst case and assume 512B is reserved.
If we don't do so, it's possible for HW to overwrite data to next
mbuf.

Signed-off-by: Chen Jing D(Mark) <jing.d.chen@intel.com>
Acked-by: Shaopeng He <shaopeng.he@intel.com>
Tested-by: Michael Qiu <michael.qiu@intel.com>
Acked-by: Michael Qiu <michael.qiu@intel.com>