fix(st-clock): print enums as unsigned

With gcc-11, the -Wformat-signedness warning complains about enum values
that should be printed as unsigned values. Change %d to %u for several
lines in the clock driver.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: Ia2d24e6feef5e852e0a6bfaa1286fe605f9a16b7