bnxt_en: fix potentially incorrect return value for ndo_rx_flow_steer

In the bnxt_en driver ndo_rx_flow_steer returns '0' whenever an entry
that we are attempting to steer is already found.  This is not the
correct behavior.  The return code should be the value/index that
corresponds to the entry.  Returning zero all the time causes the
RFS records to be incorrect unless entry '0' is the correct one.  As
flows migrate to different cores this can create entries that are not
correct.

Fixes: c0c050c5 ("bnxt_en: New Broadcom ethernet driver.")
Reported-by: Akshay Navgire <anavgire@purestorage.com>
Signed-off-by: Alex Barba <alex.barba@broadcom.com>
Signed-off-by: Andy Gospodarek <gospo@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>