greybus: svc: keep error messages uniform

All SVC error messages, except for a few recently added ones, place the
errno last after a colon (:). Let's at least try to be consistent
within the svc code.

Note that this format also allows for more concise messages without risk
for ambiguity.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>