package/lvm2: fix inverted logic that is confusing

Commit ff0f55e3 (lvm2: replace !BR2_PACKAGE_LVM2_DMSETUP_ONLY by
BR2_PACKAGE_LVM2_STANDARD_INSTALL) changed a negative-logic option to a
positive-logic option.

However, it kept the ordering of the conditional block, which became a
negatice-logic condition.

This is confusing; let's fix that.

Reported-by: Pascal de Bruijn <p.debruijn@unilogic.nl>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>