ibmvnic: Fix completion structure initialization

commit 070eca95 upstream.

Fix multiple calls to init_completion for device completion
structures. Instead, initialize them during device probe and
reinitialize them later as needed.

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