x86: asm-i386/io.h fix constness

- Fix this:

include/asm/io.h: In function `memcpy_fromio':
include/asm/io.h:208: warning: passing argument 2 of `__memcpy' discards qualifiers from pointer target type

- Clean up code a bit

Reported-by: Uwe Bugla <uwe.bugla@gmx.de>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>