This patch makes the usb-midi driver use usb_ifnum_to_if(), thereby
removing assumptions about which interface is stored in which array entry.
Similarly, it stores the bAlternateSetting value rather than the array
index for an altsetting entry.

Like the earlier patch for the audio driver, this also changes the driver
to use the in-memory rawdescriptor buffer rather than reading a
configuration descriptor from the device.  Unlike that earlier patch, this
time there's no question that the deallocation of the buffer is done
correctly.  (I suspect the audio driver just left out a call to kfree.)