RDMA/bnxt_re: Fix system crash during load/unload

During driver unload, the driver proceeds with cleanup
without waiting for the scheduled events. So the device
pointers get freed up and driver crashes when the events
are scheduled later.

Flush the bnxt_re_task work queue before starting
device removal.

Signed-off-by: Selvin Xavier <selvin.xavier@broadcom.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>