dmaengine: omap-dma: use devm_kzalloc() to allocate omap_dmadev.

Use devm_kzalloc() to allocate omap_dmadev() so that we don't need
complex error cleanup paths.

Acked-by: Tony Lindgren <tony@atomide.com>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>