lib: s390x: uv: Add UVC_ERR_DEBUG switch

Every time something goes wrong in a way we don't expect, we need to
add debug prints to some UVC to get the unexpected return code.

Let's just put the printing behind a macro so we can enable it if
needed via a simple switch.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>