serial: uartps: Fill struct uart_driver in probe()

This is preparation step for dynamic port allocation without
CDNS_UART_NR_PORTS macro. Fill the structure only once at probe.

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