bnxt_en: Add db_ring_mask and related macro to bnxt_db_info struct.

This allows the doorbell related logic to mask the doorbell index
to the proper range before writing the doorbell.

The current code masks the doorbell index immediately to keep it in the
legal ranges for the most part.  Subsequent patches will change the
logic so that the index increments unbounded and it only gets masked
before use.  This is preparation work for the new chip that requires an
additional Epoch bit in the doorbell that needs to toggle when the index
has wrapped around.

This patch just adds the basic infrastructure and the logic is largely
unchanged.  We now replace RING_CMP() with the new DB_RING_IDX() at
appropriate places where we mask the completion ring index before
writing the doorbell.

Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Link: https://lore.kernel.org/r/20231120234405.194542-10-michael.chan@broadcom.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>