make w83697hf_set_clksel_48() non-static and add a prototype

make w83697hf_set_clksel_48() non-static and add a prototype

make w83697hf_set_clksel_48() non-static and add a prototype so as to
get rid of warnings about it being unused

Change-Id: I8ae94cfd61ae4774a367f83dd37e488987e2451a
Signed-off-by: Florian Zumbiehl <florz@florz.de>
Reviewed-on: http://review.coreboot.org/380
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>