nvme/pci: Hold controller reference during async probe

It is possible the driver's remove may have freed the controller if
the remove callback is invoked prior to the async_schedule starting
the reset_work. This patch fixes that by holding a reference on the
controller.

Reported-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Keith Busch <keith.busch@intel.com>