ipmi: fix potential deadlock on &kcs_bmc->lock

As kcs_bmc_handle_event() is executed inside both a timer and a hardirq,
it should disable irq before lock acquisition otherwise deadlock could
happen if the timmer is preemtped by the irq.

Possible deadlock scenario:
aspeed_kcs_check_obe() (timer)
    -> kcs_bmc_handle_event()
    -> spin_lock(&kcs_bmc->lock)
        <irq interruption>
        -> aspeed_kcs_irq()
        -> kcs_bmc_handle_event()
        -> spin_lock(&kcs_bmc->lock) (deadlock here)

This flaw was found using an experimental static analysis tool we are
developing for irq-related deadlock.

The tentative patch fix the potential deadlock by spin_lock_irqsave()

Signed-off-by: Chengfeng Ye <dg573847474@gmail.com>
Message-Id: <20230627152449.36093-1-dg573847474@gmail.com>
Signed-off-by: Corey Minyard <minyard@acm.org>