drivers: entropy: stm32: simplify WB0-related #ifdef in LL wrappers

CONFIG_SOC_STM32WB09XX implies CONFIG_SOC_SERIES_STM32WB0X, making it
pointless to assert the latter is defined before checking the former.

Simplify #ifdef checks in the entropy driver's LL wrappers thanks to
this observation, by removing unnecessary nesting.

Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>