LVM-activate: Fix return value in lvm_status()

lvm_status() returns 0 instead of OCF_ERR_CONFIGURED if OCF_CHECK_LEVEL
has an unsupported value. This commit fixes the return value. It also
removes the case statement from its "if" nesting, adding the "0" check
as a case.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>