serial: xuartps: Fix cdns_uart_port[] definition.

The code assumes that the array cdns_uart_port[] has dimension
CDNS_UART_NR_PORTS, so let us define it this way.

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>