cxgb3: ease msi-x settings conditions

The driver currently drops to line interrupt mode
if it did not get all the msi-x vectors it requested.

Allow msi-x settings when a minimal amount of vectors
is provided.

Signed-off-by: Divy Le Ray <divy@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>