bnxt: disable napi before canceling DIM

napi schedules DIM, napi has to be disabled first,
then DIM canceled.

Noticed while reading the code.

Fixes: 0bc0b97f ("bnxt_en: cleanup DIM work on device shutdown")
Fixes: 6a8788f2

 ("bnxt_en: add support for software dynamic interrupt moderation")
Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Reviewed-by: Edwin Peer <edwin.peer@broadcom.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>