rp-pppoe: bail out if installation fails

Add an "|| exit 1" in the installation loop, otherwise if one of the
file fails to install, the build doesn't abort.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>