perf/arm-cmn: Fix invalid pointer when access dtc object sharing the same IRQ number

perf/arm-cmn: Fix invalid pointer when access dtc object sharing the same IRQ number

When multiple dtcs share the same IRQ number, the irq_friend which
used to refer to dtc object gets calculated incorrect which leads
to invalid pointer.

Fixes: 0ba64770a2f2 ("perf: Add Arm CMN-600 PMU driver")

Signed-off-by: Tuan Phan <tuanphan@os.amperecomputing.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/1623946129-3290-1-git-send-email-tuanphan@os.amperecomputing.com
Signed-off-by: Will Deacon <will@kernel.org>