perf/arm_dmc620_pmu: Fix error return code in dmc620_pmu_device_probe()

Fix to return negative error code -ENOMEM from the error handling
case instead of 0, as done elsewhere in this function.

Fixes: 53c218da

 ("driver/perf: Add PMU driver for the ARM DMC-620 memory controller")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Link: https://lore.kernel.org/r/20210312080421.277562-1-weiyongjun1@huawei.com
Signed-off-by: Will Deacon <will@kernel.org>