soc: qcom: smd: Reset rx tail rather than tx

The local end of each SMD channel is responsible for updating the tx
head and the rx tail, as such we should not touch the tx tail during a
reset.

Reported-by: Jeremy McNicoll <jmcnicol@redhat.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>