usb: chipidea: imx: fix a typo for imx6sx

Use imx6sx instead of imx6sl's platform flags for imx6sx.

Fixes: e14db48d ("usb: chipidea: imx: add runtime power management support")
Cc: <stable@vger.kernel.org> # v4.1+
Signed-off-by: Li Jun <jun.li@freescale.com>
Signed-off-by: Peter Chen <peter.chen@freescale.com>