fix(st-clock): check _clk_stm32_get_parent return

This issue was found by Coverity (CID 376885). The _clk_stm32_get_parent()
return shouldn't be negative. Return the error in this case.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I91eff7e99fcdac9a258100b163fd9b040a9bd2c0
1 file changed