bnxt_en: Add macros related to TC and TX rings

Add 3 macros that handle to conversions between TC numbers and TX
ring numbers.  These will help to clarify the existing logic and the
new logic in the next patch.

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