fix(st-clock): avoid arithmetics on pointers

This corrects MISRA C2012-18.4:
The +, -, += and -= operators should not be applied to an expression
of pointer type.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I9128f567a7c83d8e3381428b07e6bd785be2703b