ACPI / fan: Initialize acpi_state variable

Make the following compiler warning go away:

 CC      drivers/acpi/fan.o
drivers/acpi/fan.c: In function ‘fan_get_cur_state’:
drivers/acpi/fan.c:96:9: warning: ‘acpi_state’ may be used uninitialized in this function [-Wuninitialized]

by initializing the local variable acpi_state in fan_get_cur_state().

[rjw: Changelog]
Signed-off-by: Naresh Bhat <naresh.bhat@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>