usb: musb: gadget: fix compile warning

Fix the following compile warning:

drivers/usb/musb/musb_gadget.c: In function ‘rxstate’:
drivers/usb/musb/musb_gadget.c:714:22: warning: comparison of distinct pointer types lacks a cast [enabled by default]

Signed-off-by: Felipe Balbi <balbi@ti.com>