bird1: fix "Form token mismatch" errors in luci-app-bird1-ipv{4,6}

bird1: fix "Form token mismatch" errors in luci-app-bird1-ipv{4,6}

The "Filters", "Functions" and "Status" sections of the web interfaces
for both Bird4 and Bird6 threw the following error upon performing any
action:

   Form token mismatch
   The submitted security token is invalid or already expired!

Changing their entry types from "cbi" to "form" fixes them.

Fixes #922.

Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>