net: mvpp2: use correct index on array mvpp2_pools

Array mvpp2_pools is being indexed by long_log_pool, however this
looks like a cut-n-paste bug and in fact should be short_log_pool.

Detected by CoverityScan, CID#1466113 ("Copy-paste error")

Fixes: 576193f2

 ("net: mvpp2: jumbo frames support")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>