usb: musb: maintain three states for buffer mappings instead of two

If dma buffers are mapped by a higher layer, with a boolean musb_request.mapped
it is still possible to call dma_sync_single_for_device() from
musb_g_giveback(), even if txstate()/rxstate() has called unmap_dma_buffer()
before falling back to pio mode.

Moreover, check for musb_ep->dma is moved within map_dma_buffer() so where
applicable checks for it are removed. And where possible, checks for
is_dma_capable() are merged with buffer map state check.

Signed-off-by: Mian Yousaf Kaukab <mian.yousaf.kaukab@stericsson.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>