make hdlc_setup() static again

hdlc_setup was exported, but this export was never used.

If a driver using it actually shows up it can still be exported again.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Acked-by: Krzysztof Halasa <khc@pm.waw.pl>
Signed-off-by: Jeff Garzik <jeff@garzik.org>