ibmvnic: Make napi usage dynamic

In order to handle the number of rx sub crqs changing during a driver
reset, the ibmvnic driver also needs to update the number of napi.
To do this the code to init and free napi's is moved to their own
routines so they can be called during the reset process.

Signed-off-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>