rpmsg: Change complete to complete_all for open state

The open_req and open_ack completion variables are the state variables
to represet a channel remote as open. Use complete_all so there are no
races with waiters and using completion_done.

Change-Id: I073902bbbf8920534d96c9d54ac9fb16ab13614f
Signed-off-by: Chris Lew <clew@codeaurora.org>