ACPI: OSL: Make a W=1 kernel-doc warning go away

It appears that kernel-doc does not understand the return type *__ref,

drivers/acpi/osl.c:306: warning: cannot understand function prototype:
'void __iomem *__ref acpi_os_map_iomem(acpi_physical_address phys,
acpi_size size)

Signed-off-by: Qian Cai <cai@lca.pw>
[ rjw: Subject ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>