ACPI: EC: Rearrange the loop in acpi_ec_event_handler()

It is not necessary to check ec->nr_pending_queries against 0 in the
while () loop in acpi_ec_event_handler(), because that loop terminates
when ec->nr_pending_queries is 0 and the code depending on that can be
run after the loop has ended.

Modify the code accordingly and while at it rewrite the comment
regarding that code to make it clearer.

No intentional functional impact.

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