irqchip/sifive-plic: Enable interrupt if needed before EOI

RISC-V PLIC cannot "end-of-interrupt" (EOI) disabled interrupts, as
explained in the description of Interrupt Completion in the PLIC spec:

"The PLIC signals it has completed executing an interrupt handler by
writing the interrupt ID it received from the claim to the claim/complete
register. The PLIC does not check whether the completion ID is the same
as the last claim ID for that target. If the completion ID does not match
an interrupt source that *is currently enabled* for the target, the
completion is silently ignored."

Commit 69ea4630 ("irqchip/sifive-plic: Fixup EOI failed when masked")
ensured that EOI is successful by enabling interrupt first, before EOI.

Commit a1706a1c ("irqchip/sifive-plic: Separate the enable and mask
operations") removed the interrupt enabling code from the previous
commit, because it assumes that interrupt should already be enabled at the
point of EOI.

However, this is incorrect: there is a window after a hart claiming an
interrupt and before irq_desc->lock getting acquired, interrupt can be
disabled during this window. Thus, EOI can be invoked while the interrupt
is disabled, effectively nullify this EOI. This results in the interrupt
never gets asserted again, and the device who uses this interrupt appears
frozen.

Make sure that interrupt is really enabled before EOI.

Fixes: a1706a1c ("irqchip/sifive-plic: Separate the enable and mask operations")
Signed-off-by: Nam Cao <namcao@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Samuel Holland <samuel@sholland.org>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Guo Ren <guoren@kernel.org>
Cc: linux-riscv@lists.infradead.org
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20240131081933.144512-1-namcao@linutronix.de