d/rules: fix dh_auto_install

DESTDIR must have: `pwd`/debian/fw-admin
SBIN must have: `pwd`/debian/fw-admin/usr

This way, main bin installs to /usr/sbin/fw-admin, while config
files install to /etc/fw-admin.d

Signed-off-by: Arturo Borrero Gonzalez <aborrero@cica.es>