i2c: ocores: rework clk code to handle NULL cookie

For, !HAVE_CLK the clk API returns a NULL cookie. Rework the
initialization code to handle that. If clk_get_rate() delivers 0, we use
the fallback mechanisms. The patch is pretty easy when ignoring white
space issues (git diff -b).

Suggested-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Tested-by: Max Filippov <jcmvbkbc@gmail.com>