Cleanup some warning from timestamping code.

Following checkpatch.pl recommendations (which include
replacing with <linux/io.h> the <asm/io.h>, since linux/io.h includes
it).

Signed-off-by: Ezequiel Lara Gomez <ezegomez@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>