hw/i386/pc_piix: Fix RTC ISA IRQ wiring of isapc machine

hw/i386/pc_piix: Fix RTC ISA IRQ wiring of isapc machine

Commit 56b1f50e3c10 ("hw/i386/pc: Wire RTC ISA IRQs in south bridges")
attempted to refactor RTC IRQ wiring which was previously done in
pc_basic_device_init() but forgot about the isapc machine. Fix this by
wiring in the code section dedicated exclusively to the isapc machine.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2961
Fixes: 56b1f50e3c10 ("hw/i386/pc: Wire RTC ISA IRQs in south bridges")
cc: qemu-stable
Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Mark Cave-Ayland <mark.caveayland@nutanix.com>
Message-Id: <20250526203820.1853-1-shentey@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit 0b006153b7ec66505cb2d231235aa19ca5d2ce37)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>