staging: usbip: userspace: suppress a bogus error

If mkdir() of VHCI_STATE_PATH fails because the directory
already exists, that's not an error. This patch fixes
annoying "record connection" errors that would typically
come up on attach.

Signed-off-by: Ilija Hadzic <ihadzic@research.bell-labs.com>
Acked-by: David Chang <dchang@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>