luci-mod-network: fix dropdown clipping on diagnostics page

The default overflow hidden style for table cell elements breaks action
dropdowns on the diagnostics page.

Fixes: #6379
Fixes: 5f2d3054

 ("luci-mod-network: convert diagnostic page to LuCI look and feel")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>