ibmvnic: Flush existing work items before device removal

Ensure that all scheduled work items have completed before continuing
with device removal and after further event scheduling has been
halted. This patch fixes a bug where a scheduled driver reset event
is processed following device removal.

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