subsys: storage: flash_map - Use correct macro

Prior to this commit, the old `FLASH_DRIVER_NAME` macro was being used
in the definition of the flash drivers array.  This caused the array to
have a size of 0.

This commit changes the code to use the newer `FLASH_DEV_NAME` macro,
causing the configured flash device to be present in the table.

Signed-off-by: Christopher Collins <ccollins@apache.org>