usb: musb: Remove ifdefs for TX DMA for musb_host.c

We can remove the ifdefs by setting up helper functions for
mentor DMA and cppi/tusb DMA.

Note that I've kept the existing formatting as otherwise this
patch becomes pretty much unreadable.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>