drivers: clock_control: fix typo in STM32H7* clock init

Correct typo in STM32H7 HSI clock calibration directives introduced
in commit 6b167f25 ("drivers: clock_control: add calibration for
h7 pllx_hsi") that currently has no functionnal impact but is worth
to be fixed for consistency.

No functional change,

Fixes: 6b167f25
Signed-off-by: Etienne Carriere <etienne.carriere@st.com>