luci-theme-bootstrap: fix invalid dropdown state styling

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

themes/luci-theme-bootstrap/htdocs/luci-static/bootstrap/cascade.css

@@ -1300,7 +1300,7 @@ body.modal-overlay-active #modal_overlay {
 }
 
 .cbi-input-invalid,
-.cbi-input-invalid.cbi-dropdown,
+.cbi-input-invalid.cbi-dropdown:not(.btn):not(.cbi-button),
 .cbi-input-invalid.cbi-dropdown:not([open]) > ul > li,
 .cbi-value-error input {
 	color: #f00;