spi: omap2-mcspi: Fix transfers if DMADEVICES is not set

Selecting CONFIG_DMADEVICES is optional, and we must
be able to continue even without DMA. Otherwise things
like omap4430sdp nfsroot will fail if DMA is not
selected.

Note that the driver already supports PIO mode, but
we fail to fall back to PIO if requesting DMA channels
fails.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>