bnxt_en: Improve the IRQ disable sequence during shutdown.

The IRQ is disabled by writing to the completion ring doorbell.  This
should be done before the hardware completion ring is freed for correctness.
The current code disables IRQs after all the completion rings are freed.

Fix it by calling bnxt_disable_int_sync() before freeing the completion
rings.  Rearrange the code to avoid forward declaration.

Signed-off-by: Michael Chan <michael.chan@broadocm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>