usb: typec: Make sure an alt mode exist before getting its partner

Adding check to typec_altmode_get_partner() to prevent
potential NULL pointer dereference.

Reported-by: Vladimir Yerilov <openmindead@gmail.com>
Fixes: ad74b864 ("usb: typec: ucsi: Preliminary support for alternate modes")
Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>