irqchip/stm32-exti: Avoid losing interrupts due to clearing pending bits by mistake

In the current code, when the eoi callback of the exti clears the pending
bit of the current interrupt, it will first read the values of fpr and
rpr, then logically OR the corresponding bit of the interrupt number,
and finally write back to fpr and rpr.

We found through experiments that if two exti interrupts,
we call them int1/int2, arrive almost at the same time. in our scenario,
the time difference is 30 microseconds, assuming int1 is triggered first.

there will be an extreme scenario: both int's pending bit are set to 1,
the irq handle of int1 is executed first, and eoi handle is then executed,
at this moment, all pending bits are cleared, but the int 2 has not
finally been reported to the cpu yet, which eventually lost int2.

According to stm32's TRM description about rpr and fpr: Writing a 1 to this
bit will trigger a rising edge event on event x, Writing 0 has no
effect.

Therefore, when clearing the pending bit, we only need to clear the
pending bit of the irq.

Fixes: 927abfc4461e7 ("irqchip/stm32: Add stm32mp1 support with hierarchy domain")
Signed-off-by: qiuguorui1 <qiuguorui1@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: stable@vger.kernel.org # v4.18+
Link: https://lore.kernel.org/r/20200820031629.15582-1-qiuguorui1@huawei.com

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