bnxt_en: Refactor TPA logic.

Refactor the TPA logic slightly, so that the code can be more easily
extended to support TPA on the new 57500 chips.  In particular, the
logic to get the next aggregation completion is refactored into a
new function bnxt_get_agg() so that this operation is made more
generalized.  This operation will be different on the new chip in TPA
mode.  The logic to recycle the aggregation buffers has a new start
index parameter added for the same purpose.

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