ipmi_si: Avoid spurious errors for optional IRQs

Although the IRQ assignment in ipmi_si driver is optional,
platform_get_irq() spews error messages unnecessarily:
  ipmi_si dmi-ipmi-si.0: IRQ index 0 not found

Fix this by switching to platform_get_irq_optional().

Cc: stable@vger.kernel.org # 5.4.x
Cc: John Donnelly <john.p.donnelly@oracle.com>
Fixes: 7723f4c5 ("driver core: platform: Add an error message to platform_get_irq*()")
Reported-and-tested-by: Patrick Vo <patrick.vo@hpe.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Message-Id: <20200205093146.1352-1-tiwai@suse.de>
Signed-off-by: Corey Minyard <cminyard@mvista.com>