package/linux-fusion: slight simplification

Using an intermediate variable to "store" LINUX_VERSION_PROBED is
unnecessary, because they are both recursively-expanded variables, and
the `make kernel-version` code will anyway be used in both places it is
needed; storing in an intermediate variable will not make that a single
expansion of the sub-shell.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>