spi: pxa2xxx: change "no DMA channels..." msg from debug to warning

Change the type of the diagnostic message:

"no DMA channels available, using PIO"

from debug to warning.

The lack of an available DMA channel is very important regard the
spi-pxa2xx performance. The transfer speed can be reduced more than 50%.

So it is very important to warn the user about this, without enabling
the full SPI debug with CONFIG_SPI_DEBUG.

Moreover, enabling the full SPI debug only to enable this specific
debug message, the dmesg buffer fills quickly with a lot of
repetitive information during the SPI data transfer.
This cause the loss of all the first important messages
written during the initialization.

Signed-off-by: Flavio Suligoi <f.suligoi@asem.it>
Signed-off-by: Mark Brown <broonie@kernel.org>