netmap: don't use linux type struct device *

Such type cannot be used in code that is in common between
FreeBSD and Linux. Use the FreeBSD type instead.

MFC after: 3 days
Reported by: markj
Differential Revision: https://reviews.freebsd.org/D29677

(cherry picked from commit 70275a6735df8a514f48be77418491f2f8dba817)