V4L/DVB: Add notification to cxusb_dualdig4_rev2_frontend_attach() error handling

Add a notification to the dib7000p_i2c_enumeration() failure path in
cxusb_dualdig4_rev2_frontend_attach().

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>