i2c: st: Rename clock reference to something that exists

CLK_S_ICN_REG_0 hasn't existed for a while now.  This was renamed
over a few commits, then finally removed in commit 5aa02b90 (ARM:
STi: DT: STiH415: Remove unused CLK_S_ICN_REG_0 fixed clock).

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Maxime Coquelin <maxime.coquelin@st.com>
Acked-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>