usb: typec: tcpm: Error handling for tcpm_register_partner_altmodes

typec_partner_register_altmode returns ERR_PTR. Reset the pointer
altmode to NULL on failure.

Signed-off-by: Kyle Tso <kyletso@google.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20200714033453.4044482-3-kyletso@google.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>