usb: Wait after sending Set Configuration request

Some devices, like the SanDisk Cruzer Pop need some time to process
the Set Configuration request, so wait a little until they are ready.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Chin Liang See <clsee@altera.com>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Stephen Warren <swarren@nvidia.com>