mtd: pxa3xx_nand: remove unused variable 'mtd'

Remove unused variable 'mtd' to eliminate below warning.

  CC      drivers/mtd/nand/pxa3xx_nand.o
drivers/mtd/nand/pxa3xx_nand.c: In function 'pxa3xx_nand_suspend':
drivers/mtd/nand/pxa3xx_nand.c:1167: warning: unused variable 'mtd'
drivers/mtd/nand/pxa3xx_nand.c: In function 'pxa3xx_nand_resume':
drivers/mtd/nand/pxa3xx_nand.c:1180: warning: unused variable 'mtd'

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>