usb: musb: unconditionally save and restore the context on suspend

It appears not all platforms featuring a musb core need to save the musb
core registers at suspend time and restore them on resume.

The dsps platform does, however, and because it shouldn't cause any
trouble on other platforms, do it unconditionally for all of them.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>