net: mvpp2: do not update the queue mode while probing

This patch updates the probing function so that the queue mode isn't
updated while probing, as the driver would silently end up using a
configuration not wanted by the user. The patch adds an extra check to
validate the chosen queue mode instead, and the driver will fail to
probe if the configuration is invalid.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>