usb: musb: gadget: fix compilation warning

Fixes below compilation warning when musb driver is compiled for
PIO mode:

drivers/usb/musb/musb_gadget.c: In function 'musb_g_rx':
drivers/usb/musb/musb_gadget.c:840:
		warning: label 'exit' defined but not used

Signed-off-by: Ajay Kumar Gupta <ajay.gupta@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>