can: xilinx_can: Fix flags field initialization for axi can and canps

AXI CAN IP and CANPS IP supports tx fifo empty feature, this patch updates
the flags field for the same.

Signed-off-by: Appana Durga Kedareswara rao <appana.durga.rao@xilinx.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>