dp8393x: Clean up endianness hacks

According to the datasheet, section 3.4.4, "in 32-bit mode ... the SONIC
always writes long words".

Therefore, use the same technique for the 'in_use' field that is used
everywhere else, and write the full long word.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Tested-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>