Kernel: set xor min count

If iperf is started in server mode and a TCP test is performed the xor/dma
engine fails.
According to the 88F6282 datasheet the minimum byte count for XOR engine is
16 Bytes. Set the MV_XOR_MIN_BYTE_COUNT of 128 to 16.

Signed-off-by: Mario Schuknecht <schuknecht@dresearch-fe.de>