usb: musb: Fix platform code being unable to override ep access ops

musb-core was setting the ops to the default indexed or flat handlers after
checking for platform overrides. Reverse the order of this so that platform
overrides actually work.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>