net: mvpp2: update the BM buffer free/destroy logic

The buffer free routine is updated to release only given a number of
buffers, and the destroy routine now checks the actual number of buffers
in the (BPPI and BPPE) HW counters before draining the pools. This
change helps getting jumbo frames support.

Signed-off-by: Stefan Chulski <stefanc@marvell.com>
[Antoine: cosmetic cleanup, commit message]
Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>