irqchip/qcom-pdc: Drop open coded version of __assign_bit()

The driver uses what looks like an open-coded version of __assign_bit().
Replace it with the real thing.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Maulik Shah <quic_mkshah@quicinc.com>
Link: https://lore.kernel.org/r/20220224101226.88373-6-maz@kernel.org