dmaengine: failure to get a specific DMA channel is not critical

There exist systems with multiple DMA controllers with different
capabilities. For example, on some sh-mobile / rmobile systems there are
DMA controllers, whose channels can be configured to be used with
SD- and MMC-host controllers, serial ports etc. Besides there are also
DMA controllers, that can only be used for one special function, e.g.,
for USB. In such cases the DMA client filter function can just choose
to specify to the DMA driver, which channel it needs. Then the
.device_alloc_chan_resources() method of the DMA driver will check,
whether it can provide that dunction. If not, it will fail and the loop
in __dma_request_channel() will continue to the next DMA device, until
it finds a suitable one. This works fine with just one minor glitch:
the kernel logs error messages like

dmaengine: failed to get <channel name>: (-<error code>)

after each such non-critical failure. This patch lowers priority of
this message to the debug level.

Reported-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Tested-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Tested-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>