net: l2: bluetooth: Fix build break in ipsp_disconnected

In commit bf4fb51f, "net: if: Add NET_IF_UP flag"
a reference to ctxt->iface was added in the ipsp_disconnected function.

Let's add the ctxt variable definition to fix the build break.

Change-Id: Ib06047e333504f3db4fe175fb3ef1dce347e1916
Signed-off-by: Michael Scott <michael.scott@linaro.org>