usb: dwc3: omap: call of_platform_depopulate() instead

This patch fixes a bug where removing dwc3-omap.ko
would not trigger removal of dwc3.ko.

of_platform_depopulate() already bakes an easy to
use API for removing all our children which were
populated during probe(); Let's use that one instead
of cooking our own solution.

Note that this is kind of a revert of commit c5a1fbca
(usb: dwc3: dwc3-omap: Fix the crash on module removal)
although we can't simply revert that because a direct
call to platform_device_unregister would also be flakey.

Signed-off-by: Felipe Balbi <balbi@ti.com>