ARM: OMAP1: DMA: fix error handling in omap1_system_dma_init()

Add the missing iounmap() before return from omap1_system_dma_init()
in the error handling case.
Also removed platform_device_del() on add resources error case which
cause dup device delete.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Tony Lindgren <tony@atomide.com>