omap: Fix race condition in omap dma driver

The bug could cause irq enable bit of one DMA channel is
cleared/set unexpectedly when 2 (or more) drivers are calling
omap_request_dma()/omap_free_dma() simultaneously

Signed-off-by: Fei Yang <AFY095@motorola.com>
Signed-off-by: Tao Hu <taohu@motorola.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>