clk: fixed-rate: Remove export symbol on setup function

This function is only called by builtin code, but we always
exported it and had marked it as __init before commit
e4eda8e0 (clk: remove exported function from __init section,
2013-01-06) removed that marking. Given that it isn't used by
modules, lets unexport it and add back __init.

Cc: Denis Efremov <yefremov.denis@gmail.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>