usb: typec: Add plug num_altmodes sysfs attr

usb: typec: Add plug num_altmodes sysfs attr

Add a field to the typec_plug struct to record the number of available
altmodes as well as the corresponding sysfs attribute to expose this to
userspace.

This allows userspace to determine whether there are any
remaining alternate modes left to be registered by the kernel driver. It
can begin executing any policy state machine after all available
alternate modes have been registered with the connector class framework.

This value is set to "-1" initially, signifying that a valid number of
alternate modes haven't been set for the plug. The sysfs file remains
hidden as long as the attribute value is -1.

We re-use the partner attribute for number_of_alternate_modes since the
usage and name is similar, and update the corresponding *_show() command
to support both partner and plugs.

Signed-off-by: Prashant Malani <pmalani@chromium.org>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Link: https://lore.kernel.org/r/20201116201150.2919178-4-pmalani@chromium.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>