USB: cdc-wdm: no need to use usb_alloc_coherent

As Documentation/usb/dma.txt states:

  Most drivers should *NOT* be using these primitives; they don't need
  to use this type of memory (dma-coherent), and memory returned from
  kmalloc() will work just fine.

This driver handle only very low bandwith transfers.  It is not an
obvious candidate for usb_alloc_coherent().

Using these calls only serves to complicate the code for no gain,
as has been shown by multiple bugs related to this allocation path.

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>