Convert all USB gadget sysfs attributes to use the _RO or _RW variants,
to make them easier to audit and ensure that the permissions are
correct.
Note, two are left using the DEVICE_ATTR() macro, as there is no
DEVICE_ATTR_WO() in Linus's tree, that will happen after 3.12-rc1 is
out, a follow-on patch will be sent then.