RDMA/bnxt_re: Wait for delayed work to finish before device removal

Delayed work bnxt_re_worker would be still running even after
cancel_delayed_work returns. This causes crash as the driver proceeds with
device removal. To make sure that the work is finished before returning,
use cancel_delayed_work_sync.

Signed-off-by: Selvin Xavier <selvin.xavier@broadcom.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>