bnxt_en: Initialize CP doorbell value before ring allocation

The existing code does the following:
    allocate completion ring
    initialize completion ring doorbell
    disable interrupts on this completion ring by writing to the doorbell

We can have a race where firmware sends an asynchronous event to the host
after completion ring allocation and before doorbell is initialized.
When this happens driver can crash while ringing the doorbell using
uninitialized value as part of handling the IRQ/napi request.

Signed-off-by: Prashant Sreedharan <prashant.sreedharan@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>