samples: flash_shell: Use correct flash write block size

Change the flash_shell sample from using FLASH_WRITE_BLOCK_SIZE
definition to using the flash_get_write_block_size() api for the
selected flash_device.

This fixes the flash_shell sample when using multiple flash devices
with different write block sizes.

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>