rtl871x: avoid running off end of buffer

If 32 bytes of non zero are passed in pdata->pointer then the mac_pton
function will run off the end of the buffer. Make sure we always have a
terminated string kernel side.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>