gianfar: Fixup use of BUS_ID_SIZE

Commit b31a1d8b ("gianfar: Convert
gianfar to an of_platform_driver") went back to using BUS_ID_SIZE
instead of sizeof() as per the larger patch series that will remove
"char bus_id[20]" from struct device.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: David S. Miller <davem@davemloft.net>