hw/intc/apic: Fixes magic number use, removes outdated comment

hw/intc/apic: Fixes magic number use, removes outdated comment

This changes replaces the use of an explicit literal constant for
the APIC base address mask with the existing symbolic constant
intended for this purpose.

Additionally, we remove the comment about not being able to
re-enable the APIC after disabling it. This is no longer
the case after the APIC implementation's state machine was
modified in 9.0.

Signed-off-by: Phil Dennis-Jordan <phil@philjordan.eu>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20241209203629.74436-11-phil@philjordan.eu>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>