misc: liquid_dock: Defer probe until USB host is ready

The LiQUID dock driver might probe and attempt to add/remove
the USB host controller device before it has even probed.
Though not fatal, this could lead to the hub ports not being
functional when booting up initially docked. Instead, make
sure USB host controller is ready first by deferring probe
until its driver is present.

Change-Id: Icc51a9459be6f3202dcf90df632465f7d7a61914
Signed-off-by: Jack Pham <jackp@codeaurora.org>