include: drivers: flash: rts5912: fix doxygen for rts5912 flash exops

Use proper Doxygen commands so that 100% of the rts5912_flash_api_ex.h
header is documented.

Note that this uncovers weird usage of in vs. out parameters in the
various functions and might be worth a cleanup as a follow-up.

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