device: rename 'caller' field of device_sync_call_t

Rename it to 'waiter', since 'caller' is ambiguous from the point of
view of the _complete() API, since it is not the 'caller' in that case.

Change-Id: Ib3cadba99195935d67153023d731be26ffa58679
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>