USB: usbtmc: convert to use dev_groups

USB: usbtmc: 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: Guido Kiener <guido.kiener@rohde-schwarz.com>
Cc: Steve Bayless <steve_bayless@keysight.com>
Link: https://lore.kernel.org/r/20190806144502.17792-7-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>