usb: musb: Remove unnecessary shutdown function

We have remove() already calling shutdown(), so let's drop it
and move the code to remove(). No code changes, we'll drop the
the FIXME in the following patch with more clean-up.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>