ARM: 7464/1: mmc: omap_hsmmc: ensure probe returns error if DMA channel request fails

If dma_request_channel() fails (e.g. because DMA enine is not built
into the kernel), the return value from probe is zero causing the
driver to be bound to the device even though probe failed.

To fix, ensure that probe returns an error value when a DMA channel
request fail.

Signed-off-by: Kevin Hilman <khilman@ti.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>