PNP / ACPI: Do not return errors if _DIS or _SRS are not present

The ACPI PNP subsystem returns errors from pnpacpi_set_resources()
and pnpacpi_disable_resources() if the _SRS or _DIS methods are not
present, respectively, but it should not do that, because those
methods are optional.  For this reason, modify pnpacpi_set_resources()
and pnpacpi_disable_resources(), respectively, to ignore missing _SRS
or _DIS.

This problem has been uncovered by commit 202317a5 (ACPI / scan:
Add acpi_device objects for all device nodes in the namespace) and
manifested itself by causing serial port suspend to fail on some
systems.

Fixes: 202317a5 (ACPI / scan: Add acpi_device objects for all device nodes in the namespace)
References: https://bugzilla.kernel.org/show_bug.cgi?id=74371


Reported-by: wxg4net <wxg4net@gmail.com>
Reported-and-tested-by:  <nonproffessional@gmail.com>
Cc: 3.14+ <stable@vger.kernel.org> # 3.14+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>