i2c: st: fix missing struct parameter description

Fix a missing struct parameter description to allow
warning free W=1 compilation.

Signed-off-by: Alain Volmat <avolmat@me.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>