[mlir][tosa] Handle unsigned constants in `TosaConvertIntegerTypeToSignless` (#156483)

This commit fixes handling of unsigned constant data in the
`TosaConvertIntegerTypeToSignless` pass. Previously, the type of the
"values" attribute would remain unsigned, which caused an error in the
const ops verifier:
```
error: 'tosa.const' op expected same attr/result element types
  %input_zp = "tosa.const"() {values = dense<17> : tensor<1xui8>} : () -> tensor<1xui8>
              ^
note: see current operation: %0 = "tosa.const"() <{values = dense<17> : tensor<1xui8>}> : () -> tensor<1xi8>
```
Now the constant data in "values" is transformed to signless as well.
GitOrigin-RevId: 3c4ab4fdefcbd34106163899d7e2914246328616