mtd: mxc_nand: Fix warning on nr_parts unused variable

mtd: mxc_nand: Fix warning on nr_parts unused variable

If CONFIG_MTD_PARTITIONS is not selected, then the following warning is generated:

  CC      drivers/mtd/nand/mxc_nand.o
drivers/mtd/nand/mxc_nand.c: In function 'mxcnd_probe':
drivers/mtd/nand/mxc_nand.c:1014: warning: unused variable 'nr_parts'

Fix it by marking nr_parts as __maybe_unused.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>