staging: ozwpan: prevent a couple of underflows

The underflow in OZ_DATA_F_ISOC_FIXED seems not harmful, but this patch
is a clean up and makes my static checker a bit happier.

The underflow in OZ_VENDOR_CLASS_RSP seems like it could result in
memory corruption.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>