ibmvnic: change IBMVNIC_MAX_IND_DESCS to 16

The supported indirect subcrq entries on Power8 is 16. Power9
supports 128. Redefined this value to 16 to minimize the driver from
having to reset when migrating between Power9 and Power8. In our rx/tx
performance testing, we found no performance difference between 16 and
128 at this time.

Fixes: f019fb63

 ("ibmvnic: Introduce indirect subordinate Command Response Queue buffer")
Signed-off-by: Dany Madden <drt@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>