ACPI: sysfs: Prevent get_status() from returning acpi_status

The return value of get_status() is passed to user space on errors,
so it should not return acpi_status values then.  Make it return
error values that are meaningful for user space instead.

This also makes a Clang warning regarding the initialization of a
local variable in get_status() go away.

Reported-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>