serial: zynq: Write chars till output fifo is full

Change logic and put char to fifo till there is a space in output fifo.
Origin logic was that output fifo needs to be empty. It means only one
char was in output queue.
Also remove unused ZYNQ_UART_SR_TXEMPTY macro.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>