staging: atomisp: potential underflow in atomisp_get_metadata_by_type()

md_type is an enum.  On my tests, GCC treats it as unsigned but
according to the C standard it's an implementation dependant thing so we
should check for negatives.

Fixes: a49d2536 ("staging/atomisp: Add support for the Intel IPU v2")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>