USB: Remove dead references to "SAFE_SERIAL" CONFIG variables.

Remove the references to CONFIG_USBD_SAFE_SERIAL_{VENDOR,PRODUCT},
which aren't defined in any Kconfig file.

Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>