mmc: omap_hsmmc: use platform_get_resource_byname for tx/rx DMA channels

Git rid of hardcoded tx/rx DMA channels based on pdev->id
and use platform_get_resource_byname() to retrieve them
instead.

Signed-off-by: Balaji T K <balajitk@ti.com>
Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Tested-by: Venkatraman S <svenkatr@ti.com>
Signed-off-by: Chris Ball <cjb@laptop.org>