[S390] etr: fix clock synchronization race

The etr events switch-to-local and sync-check disable the synchronous clock
and schedule a work queue that tries to get the clock back into sync.
If another switch-to-local or sync-check event occurs while the work queue
function etr_work_fn still runs the eacr.es bit and the clock_sync_word can
become inconsistent because check_sync_clock only uses the clock_sync_word
to determine if the clock is in sync or not. The second pass of the
etr_work_fn will reset the eacr.es bit but will leave the clock_sync_word
intact. Fix this race by moving the reset of the eacr.es bit into the
switch-to-local and sync-check functions and by checking the eacr.es bit
as well to decide if the clock needs to be synced.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>