sfc: Remove unnecessary inclusion of <asm/io.h>, prompted by checkpatch

Fix the warning:

WARNING: Use #include <linux/io.h> instead of <asm/io.h>

There is no need for selftest.c to include the file at all.
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>

drivers/net/ethernet/sfc/selftest.c

@@ -19,7 +19,6 @@
 #include <linux/udp.h>
 #include <linux/rtnetlink.h>
 #include <linux/slab.h>
-#include <asm/io.h>
 #include "net_driver.h"
 #include "efx.h"
 #include "nic.h"