bnxt: add flag to denote that an xdp program is currently attached

This will be used to determine if bnxt_rx_xdp should be called
rather than calling it every time.

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