usb: musb: do not change dev's dma_mask

Commit 8d2421e6

 ("usb: musb: kill global and static for multi instance")
removed the global dma_mask copy and replaced by parent's DMA mask. The
problem here is that if the parent does not have a dma_mask then
musb_remove() goes kaboom.
Instead trying to fix this I was thinking we do we need to erase
dma_mask in the first place?

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>