xilinx_uartlite: Accept input after rx FIFO pop

The device return false from the can receive function when the FIFO is
full. This mean the device should check for buffered input whenever a byte is
popped from the FIFO.

Reported-by: Jason Wu <huanyu@xilinx.com>
Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
