serial: xuartps: Fix check in console_setup().

cdns_uart_console_setup() checks port->mapbase != 0, but the port may
not be initialized yet even if this condition is met [e.g., ioremap()
may have failed]. Check port->membase != NULL instead, similar to
cdns_early_console_setup().

Signed-off-by: Thomas Betker <thomas.betker@rohde-schwarz.com>
Reviewed-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>