include: drivers: flash: stm32: fix doxygen for stm32 flash exops

Use proper Doxygen commands so that 100% of the
stm32_flash_api_extensions.h header is documented.
Removed unnecessary #ifdefs since enums needed not to be guarded from
declaration in the context of the header file.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>