usb: musb: unmap dma buffer when switching to PIO

Buffer is mapped to dma when dma channel is
allocated. If, for some reason, dma channel
programming fails, musb code will fallback
to PIO mode to transfer that request. In
that case, we need to unmap the buffer
back to CPU.

MUSB RTL1.8 and above cannot handle buffers
which are not 32bit aligned. That happens to
every request sent by g_ether gadget
driver. Since the buffer sent was unaligned,
we need to fallback to PIO.

Because of that, g_ether was failing due
to missing buffer unmapping.

With this patch and [1] g_ether works fine
with all MUSB revisions.

Verified with OMAP3630 board, which has
MUSB RTL1.8 using g_ether and g_zero.

[1] http://www.spinics.net/lists/linux-usb/msg38400.html



Signed-off-by: Hema HK <hemahk@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>