bnxt_en: Set TPA GRO mode flags on 57500 chips properly.

On 57500 chips, hardware GRO mode cannot be determined from the TPA
end, so we need to check bp->flags to determine if we are in hardware
GRO mode or not.  Modify bnxt_set_features so that the TPA flags
in bp->flags don't change until the device is closed.  This will ensure
that the fast path can safely rely on bp->flags to determine the
TPA mode.

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