tty: serial: xilinx_uartps: Use Macro for ttyPS0

All instances of "ttyPS" use this macro except for this one. Convert
it.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>