ibmvnic: Fix netdev feature clobbering during a reset

While determining offload capabilities of backing hardware during
a device reset, the driver is clobbering current feature settings.
Update hw_features on reset instead of features unless a feature
is enabled that is no longer supported on the current backing device.
Also enable features that were not supported prior to the reset but
were previously enabled or requested by the user.

This can occur if the reset is the result of a carrier change, such
as a device failover or partition migration.

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