ACPI / PCI: Fix memory leak in acpi_pci_irq_enable()

acpi_pci_link_allocate_irq() can return negative gsi even if
entry != NULL.  For that case we have a memory leak, so free
entry before returning from acpi_pci_irq_enable() for gsi < 0.

Signed-off-by: Tomasz Nowicki <tomasz.nowicki@linaro.org>
Cc: All applicable <stable@vger.kernel.org>
[rjw: Subject and changelog]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>