bnxt_en: Add maximum extended request length fw message support.

Support the max_ext_req_len field from the HWRM_VER_GET_RESPONSE.
If this field is valid and greater than the mailbox size, use the
short command format to send firmware messages greater than the
mailbox size.  Newer devices use this method to send larger messages
to the firmware.

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