sam0: flash: Fix build issue reported by flash_shell sample

The DT_FLASH_DEV_NAME define used by the sample was set to a define
symbol that was never generated.  Change it to use
DT_INST_0_ATMEL_SAM0_NVMCTRL_LABEL on the sam0 family of SoCs that
had this issue.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>
4 files changed