usb: musb: tusb6010_omap: Allocate DMA channels upfront

Instead of requesting the DMA channel in tusb_omap_dma_allocate() do it
when the controller is created and in runtime work from the DMA channel
pool.

This change is needed for the DMAengine conversion of the driver since the
tusb_omap_dma_allocate() is called in interrupt context which might lead
to lock within the DMAengine API when requesting channel.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>