bnxt_en: Check the lengths of encapsulated firmware responses.

Firmware messages that are forwarded from PF to VFs are encapsulated.
The size of these encapsulated messages must not exceed the maximum
defined message size.  Add appropriate checks to avoid oversize
messages.  Firmware messages may be expanded in future specs and
this will provide some guardrails to avoid data corruption.

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