serial: zynq: Change logic in putc

serial: zynq: Change logic in putc

Sync logic with Linux kernel where TX empty flag is checked before char
is sent.
This logic is fixing problem with console on zynqmp platform.

For example:
DRAM:  2 GiB
Enabling Caches...
EL Level:	��   sdhci@ff170000: 0
Using default environment

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