nds32: constify ptr_arg

nds32: constify ptr_arg

it points to the result of strchr on a const char *, so it aliases
something that is const.  Further its only passed to a function that expects a
const char *, so there's no reason for it to not be const.

gas/ChangeLog:

2016-06-05  Trevor Saunders  <tbsaunde+binutils@tbsaunde.org>

	* config/tc-nds32.c (nds32_parse_option): Make the type of ptr_arg
	const char *.