perf/arm_cspmu: Fix event attribute type

perf/arm_cspmu: Fix event attribute type

ARM_CSPMU_EVENT_ATTR() defines a struct perf_pmu_events_attr, so
arm_cspmu_sysfs_event_show() should not be interpreting it as struct
dev_ext_attribute.

Fixes: e37dfd65731d ("perf: arm_cspmu: Add support for ARM CoreSight PMU driver")
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Reviewed-and-tested-by: Ilkka Koskinen <ilkka@os.amperecomputing.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/27c0804af64007b2400abbc40278f642ee6a0a29.1685983270.git.robin.murphy@arm.com
Signed-off-by: Will Deacon <will@kernel.org>