One of the failure paths in the arm_pmu ACPI code is missing an early
return, permitting a NULL pointer dereference upon a memory allocation
failure.

Add the missing return.

Fixes: fe40ffdb ("arm_pmu: rework ACPI probing")
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reported-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20221108093725.1239563-1-mark.rutland@arm.comSigned-off-by: Will Deacon <will@kernel.org>