USB: use sysfs_merge_group for power attributes

This patch (as1426) makes use of the new sysfs_merge_group() and
sysfs_unmerge_group() routines to simplify the handling of power
attributes for USB devices.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>