ACPI / EC: Fix returning values in acpi_ec_sync_query()

The returning value of acpi_os_execute() is erroneously handled as errno.
This patch corrects it by returning EBUSY to indicate the work queue item
creation failure.

Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>