ACPI: remove unnecessary argument checking

acpi_add_single_object() is static, and all callers supply a valid "child"
argument, so we don't need to check it.  This patch also remove some
unnecessary initializations.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Len Brown <len.brown@intel.com>