mux: mux-core: unregister mux_class in mux_exit()

Fixes an obvious and nasty typo.

Fixes: a3b02a9c ("mux: minimal mux subsystem")
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>