bnxt_en: Consolidate DB offset calculation

bnxt_en: Consolidate DB offset calculation

The doorbell offset on P5 chips is hard coded.  On the new P7 chips,
it is returned by the firmware.  Simplify the logic that determines
this offset and store it in a new db_offset field in struct bnxt.
Also, provide this offset to the RoCE driver in struct bnxt_en_dev.

Reviewed-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Signed-off-by: Hongguang Gao <hongguang.gao@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Link: https://lore.kernel.org/r/20231201223924.26955-5-michael.chan@broadcom.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>