drivers: flash: wb0x: update function to get flash size

For STM32WB0x series, LL_GetFlashSize returns the flash size in words,
not bytes. The previous implementation incorrectly interpreted this
value, leading to a large discrepancy between the expected size (512 KB)
and the computed size (64 MB) in the test_get_size ztest.

This change updates the logic to correctly compute the flash size in
bytes, ensuring accurate results in size-related operations.

Signed-off-by: Fabrice DJIATSA <fabrice.djiatsa-ext@st.com>