rpmsg: smd: Reduce restrictions when finding channel

SMD channels are created by the remotes in "opening" state, but
sometimes as we close and try to reopen them they linger in closing
state.

Following the search for a matching channel the create_ept() will verify
that the channel is in a suitable state, so we can lax the restrictions
of the search function to work around above difference in behaviour.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>