sfc: check async completer is !NULL before calling

Add a NULL check before calling asynchronous MCDI completion functions
during device removal.

Fixes: 7014d7f6

 ("sfc: allow asynchronous MCDI without completion function")
Signed-off-by: Bert Kenward <bkenward@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>