drivers: serial: xuartps: fixing set_baudrate function

The Xilinx xuartps driver fails to find the correct pre-scaler
parameters for the BAUDDIV and BAUDGEN registers. This fix
corrects that behaviour. The reason why it fails is due to
the stop condition for the search being ill-conditioned.
The stop condition is based on a baudrate error, and this
was chosen to be 3 percent. This would cause the parameter
search to stop too early returning ill-condotioned pre-scaler
parameters. This has been corrected by setting the stop condition
to be 3 permille instead of 3 percent. This change was tested
with an AMD Zynq Ultrascale+ MPSoC ZU3EG.

Signed-off-by: Kent Stark Olsen <kent.stark.olsen@gmail.com>