ACPI: drop unnecessary local variable from acpi_system_write_wakeup_device()

The LEN variable is unsigned, therefore checking whether it is less than 0 is
useless. Also drop the LEN variable, since the COUNT parameter can be used
instead.

[rjw: Changed the subject.]
Signed-off-by: Cyril Roelandt <tipecaml@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>