soc: stm32wb: fix ble low-power

Release HSI CLK48 semaphore when going to sleep to allow C2 (M0)
core to start and stop clock as needed while C1 core is not running.

CLK48 is shared between RNG and USB. RNG is needed by M0 during BLE
advertisement. If semaphore is locked, C2 core can start it when it
needs to but not stop it.

Fixes zephyrproject-rtos#69955.

Signed-off-by: Jonny Gellhaar <jonny.gellhaar@prevas.se>