net: smc91x: take into account half-word workaround

For device-tree builds, platforms such as mainstone, idp and stargate2
must have their u16 writes all aligned on 32 bit boundaries. This is
already enabled in platform data builds, and this patch adds it to
device-tree builds.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>