The logic to allow only one DMA driver in MUSB is currently
flawed, because it also allows picking no DMA driver at all
and also not selecting PIO mode.

Using a choice statement makes this foolproof for now and
also simplifies the Makefile.

Unfortunately, we will have to revisit this when we start
supporting multiple ARM platforms in a single kernel binary,
because at that point we will actually need to select
multiple DMA drivers and pick the right one at run-time.
Signed-off-by: NArnd Bergmann <arnd@arndb.de>
Cc: Felipe Balbi <balbi@ti.com>
Signed-off-by: NFelipe Balbi <balbi@ti.com>