net: mvneta: increase number of buffers in RX and TX queue

The initial values were too small leading to poor performance when using
the software buffer management.

Signed-off-by: Yelena Krivosheev <yelena@marvell.com>
[gregory: extract from a larger patch]
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>