net: mvpp2: avoid checking for free aggregated descriptors twice

Avoid repeating the check for free aggregated descriptors when it
already failed at the beginning of the function.

Signed-off-by: Yan Markman <ymarkman@marvell.com>
[Antoine: commit message]
Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>