drivers: serial: ns16550: Condition added for dma_callback

Enable a condition as define dma_callback function only if
any one instance of ns16550 has dmas parameter in dts.
This resolves conflict of dma_callback function defined but
not used warning in case of UART_ASYNC_API enabled but dmas
parameter is not provided to any ns16550 UARTs dts instances.

Signed-off-by: Anisetti Avinash Krishna <anisetti.avinash.krishna@intel.com>