usb: musb: use a Kconfig choice to pick the right DMA method

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: Arnd Bergmann <arnd@arndb.de>
Cc: Felipe Balbi <balbi@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>