usb: typec: thunderbolt: Remove IS_ERR check for plug

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 <bleung@chromium.org>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Link: https://lore.kernel.org/r/Z5PstnlA52Z1F2SU@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>