musb: Export musb_root_disconnect for use in modules

Export musb_root_disconnect for use in modules, so that musb glue
code build as module can use it.

This fixes the buildbot errors for -next in arm64-allmodconfig
and arm-allmodconfig.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Fixes: 7cba17ec ("musb: sunxi: Add support for platform_set_mode")
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>