memory: omap-gpmc: Fix build with CONFIG_OMAP_GPMC disabled

Fix the following build failure if omap-gpmc.h is
used with CONFIG_OMAP_GPMC disabled.

./include/linux/omap-gpmc.h:32:1: error: unknown type name ‘gpmc_nand_ops’
Signed-off-by: Roger Quadros <rogerq@ti.com>

include/linux/omap-gpmc.h

@@ -29,8 +29,8 @@ struct gpmc_nand_regs;
 struct gpmc_nand_ops *gpmc_omap_get_nand_ops(struct gpmc_nand_regs *regs,
 					     int cs);
 #else
-static inline gpmc_nand_ops *gpmc_omap_get_nand_ops(struct gpmc_nand_regs *regs,
-						    int cs)
+static inline struct gpmc_nand_ops *gpmc_omap_get_nand_ops(struct gpmc_nand_regs *regs,
+							   int cs)
 {
 	return NULL;
 }