usbip: change dev_attr_group to constant

The dev_attr_group variable is never changed and it is only passed
to the second parameter of sysfs_create_group() and sysfs_remove_group() functions.
These functions are defined in linux/sysfs.h: the second parameter is a pointer to
const in both cases.

Signed-off-by: Márton Németh <nm127@freemail.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>