usb: musb: patches for v3.6 merge window

Just two patches here:

First we have a fix to disable DMA in case DMA channel
request fails.

Second, there's a fix for situations where the user
kills musb (by rmmod or any other means) while a
transfer is on the fly. In such cases, we could be
led into a NULL pointer dereference due to endpoint
being disabled and endpoint descriptor being NULL.