netmap: Stop printing a line to the dmesg in netmap_init()

netmap is compiled into the kernel by default so initialization was
always reported, and netmap uses a formatting convention not used in the
rest of the kernel.

Reviewed by: vmaffione
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29099

(cherry picked from commit fef845097190f0ecb783d6c75a9398c4e4a4c0e1)