bnxt_en: Prevent kernel panic when receiving unexpected PHC_UPDATE event

The firmware can send PHC_RTC_UPDATE async event on a PF that may not
have PTP registered. In such a case, there will be a null pointer
deference for bp->ptp_cfg when we try to handle the event.

Fix it by not registering for this event with the firmware if !bp->ptp_cfg.
Also, check that bp->ptp_cfg is valid before proceeding when we receive
the event.

Fixes: 8bcf6f04 ("bnxt_en: Handle async event when the PHC is updated in RTC mode")
Signed-off-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>