staging: most: make interface drivers allocate coherent memory

On arm64/aarch64 architectures the allocation of coherent memory needs a
device that has the dma_ops properly set. That's why the core module of
the MOST driver is no longer able to allocate this type or memory. This
patch moves the allocation process down to the interface drivers where
the proper devices exist (e.g. platform device or USB system software).

Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>