USB: usbip: convert to use dev_groups

USB drivers now support the ability for the driver core to handle the
creation and removal of device-specific sysfs files in a race-free
manner.  Take advantage of that by converting the driver to use this by
moving the sysfs attributes into a group and assigning the dev_groups
pointer to it.

Cc: Valentina Manea <valentina.manea.m@gmail.com>
Cc: Shuah Khan <shuah@kernel.org>
Link: https://lore.kernel.org/r/20190806144502.17792-13-gregkh@linuxfoundation.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>