ibmvnic: Introduce driver limits for ring sizes

Introduce driver-defined maximums for queue ring sizes. Devices
available for IBM vNIC today will likely not allow this amount,
but this should give us some leeway for future devices that may
support larger ring sizes.

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