After enabling the EC event handling, Linux is still in the noirq stage, if
there is no triggering source (EC transaction, GPE STS status),
advance_transaction() will not be invoked and SCI_EVT cannot be detected.
This patch adds one more triggering source after enabling the EC event
handling to poll the pending SCI_EVT.

Known issues:
1. Still no SCI_EVT triggering source
   There could still be no SCI_EVT triggering source after handling the
   first SCI_EVT (polled by this patch if any). Because after handling the
   first SCI_EVT, Linux could still be in noirq stage and there could still
   be no further triggering source in this stage. Then the second SCI_EVT
   indicated during this stage still cannot be detected by the EC driver.
   With this improvement applied, it is then possible to move
   acpi_ec_enable_event() out of the noirq stage to fix this issue (if the
   first SCI_EVT is handled out of the noirq stage, the follow-up SCI_EVTs
   should be able to trigger IRQs).
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Tested-by: Todd E Brandt <todd.e.brandt@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>