usb: typec: Fix htmldocs warning

Fix htmldocs warning:

drivers/usb/typec/mux.c:186: warning: Function parameter or member 'mux' not described in
'typec_mux_unregister'

Reported-by: kbuild test robot <lkp@intel.com>
Fixes: bdecb33a ("usb: typec: API for controlling USB Type-C Multiplexers")
Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>