net/bnxt: fix interrupt handler

In some cases the interrupt handler is accessing cpr, which has
already been freed causing segfaults. This patch avoids such accesses.

Fixes: 7bc8e9a227cc ("net/bnxt: support async link notification")
Cc: stable@dpdk.org
Signed-off-by: Ajit Khaparde <ajit.khaparde@broadcom.com>