fix NULL pointer with MUSB and DMA on ISO ep

For the case of an isochonous endpoint, musb_write_fifo may be
called with a NULL buffer from musb_host_tx if DMA is enabled.
Remove DMA check and always supply a valid buffer as long as
the URB contains a valid buffer.

Signed-off-by: Hunyue Yau <hyau@mvista.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>