bnxt_en: Use macros to define port statistics size and offset.

The port statistics structures have hard coded padding and offset.
Define macros to make this look cleaner.

Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Reviewed-by: Vasundhara Volam <vasundhara-v.volam@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>