usb: gadget: udc: fix free_irq() after request_irq() failed

My static checker detected the mistake. I fix this by changing "goto
err_irq" to "goto err_req". The label err_irq is not used now
so this patch also removes it.

Signed-off-by: Takeshi Yoshimura <yos@sslab.ics.keio.ac.jp>
Signed-off-by: Felipe Balbi <balbi@ti.com>