Refactor iwmbtfw: set default firmware_dir after argument parsing

Don't repeat firmware_dir stanza at all "if" branches: be more DRY.

Signed-off-by: Eygene Ryabinkin <rea@FreeBSD.org>
Reviewed by: wulf
MFC after: 1 month

(cherry picked from commit f466ba4ca479ec500b927deadaba104469662994)