MFC r286887: Using the error return code documented in the comment.

Though there is no direct midi_uninit() caller amongst existing drivers
at this moment, a quick experiment indicates that EBUSY gives users more
precise error message once drivers start to honour this result. For example,
emu_midi_detach() should check the result of mpu401_uninit() and block
module unloading if there is any MIDI I/O in progress.