drivers: clock_control: stm32: Use hclk freq for flash latency computation

Flash is clocked with HCLK, while CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC
reflects SYSCLK. HCLK = SYCLK / AHB prescaler.
When dealing with flash latency, use HCLK instead of SYSCLK.

This changes reverts a abusive change done in an old commit (efd8ee46)

Signed-off-by: Erwan Gouriou <erwan.gouriou@st.com>