USB: usbip: fix nonconforming hub descriptor

Fix up the root-hub descriptor to accommodate the variable-length
DeviceRemovable and PortPwrCtrlMask fields, while marking all ports as
removable (and leaving the reserved bit zero unset).

Also add a build-time constraint on VHCI_HC_PORTS which must never be
greater than USB_MAXCHILDREN (but this was only enforced through a
KConfig constant).

This specifically fixes the descriptor layout whenever VHCI_HC_PORTS is
greater than seven (default is 8).

Fixes: 04679b34 ("Staging: USB/IP: add client driver")
Cc: Takahiro Hirofuchi <hirofuchi@users.sourceforge.net>
Cc: Valentina Manea <valentina.manea.m@gmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Acked-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>