usb: typec: Make the attributes read-only when writing is not possible

usb: typec: Make the attributes read-only when writing is not possible

This affects the read-writable attribute files. Before this
there was no way for the user to know is changing the value
supported or not.

>From now on those attribute files will be made read-only
unless the underlying driver supports changing of the value.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Link: https://lore.kernel.org/r/20200211112531.86510-2-heikki.krogerus@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>