net: smsc91x: add u16 workaround for pxa platforms

Add a workaround for mainstone, idp and stargate2 boards, for u16 writes
which must be aligned on 32 bits addresses.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Cc: Jeremy Linton <jeremy.linton@arm.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>