net: smc91x: isolate u16 writes alignment workaround

Writes to u16 has a special handling on 3 PXA platforms, where the
hardware wiring forces these writes to be u32 aligned.

This patch isolates this handling for PXA platforms as before, but
enables this "workaround" to be set up dynamically, which will be the
case in device-tree build types.

This patch was tested on 2 PXA platforms : mainstone, which relies on
the workaround, and lubbock, which doesn't.

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