net: ehea: convert to use DRIVER_ATTR_RO

We are trying to get rid of DRIVER_ATTR(), and the ehea driver's
attribute can be trivially changed to use DRIVER_ATTR_RO().

Cc: Douglas Miller <dougmill@linux.vnet.ibm.com>
Cc: <netdev@vger.kernel.org>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>