bnxt_en: Refactor bnxt_tx_int()

bnxt_tx_int() processes the only one TX ring from the bnxt_napi pointer.
To prepare for more TX rings associated with the bnxt_napi structure,
add a new __bnxt_tx_int() function that takes the bnxt_tx_ring_info
pointer to process that one TX ring.  No functional change.

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>