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 <artem.bityutskiy@linux.intel.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>