frontend: Fix css on Firefox 24.5

Fixed the positioning of the checkbox label in the
SerialNumberPolicyWidget.

Change-Id: I51cba6e0e4fe165f99e7638652e6f1286f68318c
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1115326
Signed-off-by: Martin Betak <mbetak@redhat.com>