usb: typec: ucsi: Fix null pointer dereference in trace

In the Linux kernel, the following vulnerability has been resolved:

usb: typec: ucsi: Fix null pointer dereference in trace

ucsi_register_altmode checks IS_ERR for the alt pointer and treats

NULL as valid. When CONFIG_TYPEC_DP_ALTMODE is not enabled,

ucsi_register_displayport returns NULL which causes a NULL pointer

dereference in trace. Rather than return NULL, call

typec_port_register_altmode to register DisplayPort alternate mode

as a non-controllable mode when CONFIG_TYPEC_DP_ALTMODE is not enabled.