usb: chipidea: fix static checker warning for NULL pointer

During the static checker, "data->usbmisc_data" may be NULL.
Fix it by adding this pointer judgement before using.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Peter Chen <peter.chen@nxp.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>