usb: gadget: udc: update outdated comment for renamed usb_gadget_udc_start()

usb: gadget: udc: update outdated comment for renamed usb_gadget_udc_start()

The function usb_gadget_udc_start() was renamed to
usb_gadget_udc_start_locked() by commit 286d9975a838 ("usb: gadget:
udc: core: Prevent soft_connect_store() race").  Update the comment
in usb_gadget_udc_set_speed() accordingly.

Assisted-by: unnamed:deepseek-v3.2 coccinelle
Signed-off-by: Kexin Sun <kexinsun@smail.nju.edu.cn>
Link: https://patch.msgid.link/20260321110006.8484-1-kexinsun@smail.nju.edu.cn
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>