usb: gadget: fsl_udc_core: fix compile error.

Fix compile error in file drivers/usb/gadget/fsl_udc_core.c.

drivers/usb/gadget/fsl_udc_core.c: In function 'portscx_device_speed':
drivers/usb/gadget/fsl_udc_core.c:1720: error: 'speed' undeclared (first
use in this function)

Introduced in commit e538dfdae85244fd2c4231725d82cc1f1bc4942c
(usb: Provide usb_speed_string() function)

Signed-off-by: Alexander Aring <a.aring@phytec.de>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>