irqchip/sifive-plic: Fix broken irq_set_affinity() callback

An interrupt submitted to an affinity change will always be left enabled
after plic_set_affinity() has been called, while the expectation is that
it should stay in whatever state it was before the call.

Preserving the configuration fixes a PWM hang issue on the Unleashed
board.

[  919.015783] rcu: INFO: rcu_sched detected stalls on CPUs/tasks:
[  919.020922] rcu:     0-...0: (0 ticks this GP)
idle=7d2/1/0x4000000000000002 softirq=1424/1424 fqs=105807
[  919.030295]  (detected by 1, t=225825 jiffies, g=1561, q=3496)
[  919.036109] Task dump for CPU 0:
[  919.039321] kworker/0:1     R  running task        0    30      2 0x00000008
[  919.046359] Workqueue: events set_brightness_delayed
[  919.051302] Call Trace:
[  919.053738] [<ffffffe000930d92>] __schedule+0x194/0x4de
[  982.035783] rcu: INFO: rcu_sched detected stalls on CPUs/tasks:
[  982.040923] rcu:     0-...0: (0 ticks this GP)
idle=7d2/1/0x4000000000000002 softirq=1424/1424 fqs=113325
[  982.050294]  (detected by 1, t=241580 jiffies, g=1561, q=3509)
[  982.056108] Task dump for CPU 0:
[  982.059321] kworker/0:1     R  running task        0    30      2 0x00000008
[  982.066359] Workqueue: events set_brightness_delayed
[  982.071302] Call Trace:
[  982.073739] [<ffffffe000930d92>] __schedule+0x194/0x4de
[..]

Fixes: bb0fed1c ("irqchip/sifive-plic: Switch to fasteoi flow")
Signed-off-by: Greentime Hu <greentime.hu@sifive.com>
[maz: tidy-up commit message]
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20201020081532.2377-1-greentime.hu@sifive.com