mmc: omap_hsmmc: convert to dma_request_slave_channel_compat

Convert dmaengine channel requests to use
dma_request_slave_channel_compat(). This supports platforms booting
with or without DT populated.

Signed-off-by: Matt Porter <mporter@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Balaji T K <balajitk@ti.com>
Signed-off-by: Chris Ball <cjb@laptop.org>