drivers: flash: stm32: always define flash_stm32_flush_caches

This reduces the amount of #ifdef and keep all of them at the same
place. The empty function is then optimized-out by the compiler.

At the same time change the negative test by a positive one to simplify
adding new entries.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>