dpaa2-eth: Update buffer pool refill threshold

Add more buffers to the Rx buffer pool as soon as 7 of them
get consumed, instead of waiting for their number to drop
below a fixed threshold.
7 is the number of buffers that can be released in the pool
via a single DPIO command.

Signed-off-by: Ioana Radulescu <ruxandra.radulescu@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>