bnxt_en: Refactor bnxt_poll_work().

Separate the CP ring polling logic in bnxt_poll_work() into 2 separate
functions __bnxt_poll_work() and __bnxt_poll_work_done().  Since the logic
is separated, we need to add tx_pkts and events fields to struct bnxt_napi
to keep track of the events to handle between the 2 functions.  We also
add had_work_done field to struct bnxt_cp_ring_info to indicate whether
some work was performed on the CP ring.

This is needed to better support the 57500 chips.  We need to poll up to
2 separate CP rings before we update or ARM the CP rings on the 57500 chips.

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