of: mtd: nuke useless const qualifier

This patch does the following:
-const int of_get_nand_ecc_mode(struct device_node *np)
+int of_get_nand_ecc_mode(struct device_node *np)

because:
1. it is probably just a typo?
2. it causes warnings like this when people assing the returned
value to an 'int' variable:
include/linux/of_mtd.h:14:18: warning: type qualifiers ignored on functi=
on return type [-Wignored-qualifiers]

Remove also the unnecessary "extern" qualifier to be consistent with other
declarations in this file.

Signed-off-by: Artem Bityutskiy <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>

drivers/of/of_mtd.c

···
  * The function gets ecc mode string from property 'nand-ecc-mode',
  * and return its index in nand_ecc_modes table, or errno in error case.
  */
-const int of_get_nand_ecc_mode(struct device_node *np)
+int of_get_nand_ecc_mode(struct device_node *np)
 {
 	const char *pm;
 	int err, i;

include/linux/of_mtd.h

···
 
 #ifdef CONFIG_OF_MTD
 #include <linux/of.h>
-extern const int of_get_nand_ecc_mode(struct device_node *np);
+int of_get_nand_ecc_mode(struct device_node *np);
 int of_get_nand_bus_width(struct device_node *np);
 bool of_get_nand_on_flash_bbt(struct device_node *np);
 #endif