luci-mod-system: flash.js: fix mtdname on download

The previous change did not work as intended for
partitions with _ in the name. I.e "0:qsee_1".
It would output an error "Invalid characters in filename".

So fix this by matching and replacing any
character except for a-z and 0-9 with "-".

Example "0:qsee_1" = I.e "0-qsee-1".

Signed-off-by: Kristian Skramstad <kristian+github@83.no>

modules/luci-mod-system/htdocs/luci-static/resources/view/system/flash.js

@@ -172,7 +172,7 @@ return view.extend({
 	handleBlock: function(hostname, ev) {
 		var mtdblock = dom.parent(ev.target, '.cbi-section').querySelector('[data-name="mtdselect"] select');
 		var mtdnumber = mtdblock.value;
-		var mtdname = mtdblock.selectedOptions[0].text;
+		var mtdname = mtdblock.selectedOptions[0].text.replace(/([^a-zA-Z0-9]+)/g, '-');
 		var form = E('form', {
 			'method': 'post',
 			'action': L.env.cgi_base + '/cgi-download',