Allow musb_platform_enable to return an error code and propagate it up to
usb_lowlevel_init().

This allows moving the checks for an external vbus being present to be
moved from platform_init to platform_enable, so that the user can unplug a
charger, plug in a host adapter with a usb-device, do a "usb reset" and
have things working.

This also allows adding a check for the id-pin to platform_enable, so that
it can short circuit the 1s delay in usb_lowlevel_init() when no host cable
is plugged in and thus waiting for a device to show up is useless.

Note that all the changes to code shared with the kernel are wrapped in
the kernel.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Simon Glass <sjg@chromium.org>