mtd: atmel_nand: fix warning: 'part_probes' defined but not used

This patch fixes

drivers/mtd/nand/atmel_nand.c:372: warning: 'part_probes' defined but not used

which is issued when CONFIG_MTD_PARTITIONS is defined but
CONFIG_MTD_CMDLINE_PARTS not.

Signed-off-by: Andreas Bießmann <biessmann@corscience.de>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>