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 e538dfda
(usb: Provide usb_speed_string() function)
Signed-off-by: NAlexander Aring <a.aring@phytec.de>
Acked-by: NMichal Nazarewicz <mina86@mina86.com>
Signed-off-by: NFelipe Balbi <balbi@ti.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>