usb: typec: mux: Get the mux identifier from function parameter

In order for the muxes to be usable with alternate modes,
the alternate mode devices will need also to be able to get
a handle to the muxes on top of the port devices. To make
that possible, the muxes need to be possible to request with
an identifier.

This will change the API so that the mux identifier is given
as a function parameter to typec_mux_get(), and the hard-coded
"typec-mux" is replaced with that value.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Tested-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>