uqmi: wait for the control device too

uqmi: wait for the control device too

The control device /dev/cdc-wdm0 is not available immediately on the
D-Link DWR-921 Rev.C3, therefore the wwan interface fails to start at
boot with a "The specified control device does not exist" error.

This patch alters /lib/netifd/proto/qmi.sh to wait for
network.wwan.delay earlier, before checking for the control device,
instead of just before interacting with the modem.

One still has to use network.wwan.proto='qmi', as the "wwan" proto
performs that sort of check before any delay is possible, failing with a
"No valid device was found" error.

Signed-off-by: Thomas Equeter <tequeter@users.noreply.github.com>