bnxt_en: Fix regressions when setting up MQPRIO TX rings.

Recent changes added the bnxt_init_int_mode() call in the driver's open
path whenever ring reservations are changed.  This call was previously
only called in the probe path.  In the open path, if MQPRIO TC has been
setup, the bnxt_init_int_mode() call would reset and mess up the MQPRIO
per TC rings.

Fix it by not re-initilizing bp->tx_nr_rings_per_tc in
bnxt_init_int_mode().  Instead, initialize it in the probe path only
after the bnxt_init_int_mode() call.

Fixes: 674f50a5 ("bnxt_en: Implement new method to reserve rings.")
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>