usb: gadget: udc: renesas_usb3: Fix sysfs interface of "role"

Since the role_store() uses strncmp(), it's possible to refer
out-of-memory if the sysfs data size is smaller than strlen("host").
This patch fixes it by using sysfs_streq() instead of strncmp().

Fixes: cc995c9e

 ("usb: gadget: udc: renesas_usb3: add support for usb role swap")
Cc: <stable@vger.kernel.org> # v4.12+
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>