remoteproc: sysmon: Improve error messages

Improve the style of a few of the error messages printed by the sysmon
implementation and fix the copy-pasted shutdown error in the send-event
function.

Tested-by: Steev Klimaszewski <steev@kali.org>
Reviewed-by: Rishabh Bhatnagar <rishabhb@codeaurora.org>
Link: https://lore.kernel.org/r/20201122054135.802935-5-bjorn.andersson@linaro.org


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