zephyr: remove deprecated DT_CHOSEN_ZEPHYR_FLASH_CONTROLLER_LABEL

DT_CHOSEN_ZEPHYR_FLASH_CONTROLLER_LABEL is going to be deprecated, so
remove its usages from the Zephyr port. Definition checks have been
replaced with DT_HAS_CHOSEN(zephyr_flash_controller), and the macro
itself has been replaced by
DT_LABEL(DT_CHOSEN(zephyr_flash_controller)). Note that the code could
likely be refactored to make use of compile time device references, ie
use DEVICE_DT_GET, but that task has been left for the maintainers.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>