linux-user: change do_semop to return target errno when unsuccessful

linux-user: change do_semop to return target errno when unsuccessful

do_semop() is called from two places, and one of these fails to convert
return error to target errno when semop fails. This patch changes the
function to always return target errno in case of an unsuccessful call.

Signed-off-by: Petar Jovanovic <petar.jovanovic@imgtec.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>