Revert const device in discover_client_ops

Revert const device in discover_client_ops

Make the device structure passed to the discover_client_ops callbacks
writable.

Commit 2b42cdd35ccd83c52e9df257efdfcda040d170d9 (Make client ops constant)
changed the discover_client_ops callback routine arguments to be pointers
to const device structures.  Typically, the callback routines will
initialize and destroy the device.ui_info and boot_option.info variables,
so the device instance must be writable by the callback routines.

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>