bnxt_en: cleanup DIM work on device shutdown

Make sure to cancel any pending work that might update driver coalesce
settings when taking down an interface.

Fixes: 6a8788f2 ("bnxt_en: add support for software dynamic interrupt moderation")
Signed-off-by: Andy Gospodarek <gospo@broadcom.com>
Cc: Michael Chan <michael.chan@broadcom.com>
Acked-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>