usb: typec: thunderbolt: Remove IS_ERR check for plug

Fixes these Smatch static checker warnings:
drivers/usb/typec/altmodes/thunderbolt.c:354 tbt_ready() warn: 'plug' is not an error pointer

Fixes: 100e25738659 ("usb: typec: Add driver for Thunderbolt 3 Alternate Mode")
Signed-off-by: Benson Leung <[e-mail]>
Reported-by: Dan Carpenter <[e-mail]>
Reviewed-by: Heikki Krogerus <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/typec/altmodes/thunderbolt.c

···
 	 */
 	for (int i = 0; i < TYPEC_PLUG_SOP_PP + 1; i++) {
 		plug = typec_altmode_get_plug(tbt->alt, i);
-		if (IS_ERR(plug))
+		if (!plug)
 			continue;
 
-		if (!plug || plug->svid != USB_TYPEC_TBT_SID)
+		if (plug->svid != USB_TYPEC_TBT_SID)
 			break;
 
 		plug->desc = "Thunderbolt3";