kern_conf: fully initialize cloned devices with make_dev_args, too

Attempting to initialize si_drv{1,2} with mda_si_drv{1,2} does not work if
you are operating on cloned devices.

clone_create must be called prior to the make_dev* family to create/return
the device on the clonelist as needed. This device is later returned early
in newdev(), prior to si_drv{0,1,2} initialization.

This patch simply breaks out of the loop if we've found a device and
finishes init.

Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D21904