serial: sh-sci: suppress warning for ports without dma channels

If a port has no dma channel defined in the device tree, then
don't attempt to allocate a dma channel for the port.
Also suppress the warning message concerning the failure to allocate
a dma channel.  Continue to emit the warning message if a dma
channel is defined but cannot be allocated.

Signed-off-by: Andy Lowe <andy_lowe@mentor.com>
Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>