luci-app-attendedsysupgrade: clear request hash before every build request

Since commit b2ad5d724ef8d0f248080109b7098626e5b5c40c enables you to do multiple searches for a firmware upgrade without reloading the page, it was discovered that if the request hash is not reset, you will get the previous build with that specific request hash as the response even if you select new packages.

Signed-off-by: Daniel Nilsson <daniel.nilsson94@outlook.com>