bnxt_en: Use macros for firmware message doorbell offsets.

In preparation for adding a 2nd communication channel to firmware.

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