eudev: fix printf usage in init script

Using a variable in a printf format string may lead to undesirable
results if the variable contains format controls, so replace

    printf "foo $var bar"

by

    printf "foo %s bar" "$var"

Signed-off-by: Carlos Santos <casantos@datacom.ind.br>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>