usb: phy: Initialize struct usb_phy list_head

As part of the registration of a new 'struct usb_phy' with the USB PHY core
via either usb_add_phy(struct usb_phy *x, ...) or usb_add_phy_dev(struct
usb_phy *x) these functions call list_add_tail(&x->head, phy_list) in
order for the new instance x to be stored in phy_list, a static list
kept internally by the core.

After 7d21114dc6a2 ("usb: phy: Introduce one extcon device into usb phy")
when executing either of the registration functions above it is possible
that usb_add_extcon() fails, leading to either function returning before
the call to list_add_tail(), leaving x->head uninitialized.

Then, when a driver tries to undo the failed registration by calling
usb_remove_phy(struct usb_phy *x) there will be an unconditional call to
list_del(&x->head) acting on an uninitialized variable, and thus a
possible NULL pointer dereference.

Fix this by initializing x->head before usb_add_extcon() has a
chance to fail. Note that this was not needed before 7d21114dc6a2 since
list_add_phy() was executed unconditionally and it guaranteed that x->head
was initialized.

Fixes: 7d21114dc6a2 ("usb: phy: Introduce one extcon device into usb phy")
Cc: stable <[e-mail]>
Signed-off-by: Diogo Ivo <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/phy/phy.c

···
 		return -EINVAL;
 	}
 
+	INIT_LIST_HEAD(&x->head);
+
 	usb_charger_init(x);
 	ret = usb_add_extcon(x);
 	if (ret)
···
 		dev_err(x->dev, "no device provided for PHY\n");
 		return -EINVAL;
 	}
+
+	INIT_LIST_HEAD(&x->head);
 
 	usb_charger_init(x);
 	ret = usb_add_extcon(x);