[PATCH] USB: net2280 warning fix

For some reason alpha doesn't include <linux/dma-mapping.h> where other
architectures do; this makes net2280 include it explicitly.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>