ibmvnic: Increase maximum queue size limit

Increase queue size limit to 16. Devices available for IBM vNIC today
will not allow this amount, but this should give us some leeway for
future devices that may support more RX or TX queues.

Signed-off-by: Thomas Falcon <tlfalcon@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>