usb: musb: core: fix possible build error with randconfig

when making commit e574d57 (usb: musb: fix
compile warning) I forgot to git add this
part of the patch which ended up introducing
a possible build error.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>