bnxt_en: Do not send firmware messages if firmware is in error state.

Add a flag to mark that the firmware has encountered fatal condition.
The driver will not send any more firmware messages and will return
error to the caller.  Fix up some clean up functions to continue
and not abort when the firmware message function returns error.

This is preparation work to fully handle firmware error recovery
under fatal conditions.

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