fwcontrol: eliminate set but not used warning

The variable was used in an #if 0 block; just move the variable
definition and setting into the same block since Firewire is mainly of
historical interest and is unlikely to see ongoing development in
FreeBSD.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 2d015cffc73ab1ff0e8be9004ab89ecbc777af78)