soc: qcom: smp2p: Correct addressing of outgoing value

The valid_entries index should not be incremented until after we have
acquired the pointer to the value, or we will read and write data one
item off.

Fixes: 50e99641 ("soc: qcom: smp2p: Qualcomm Shared Memory Point to Point")
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>