i2c: completion header cleanups

i2c-core and i2c-isa use completions without including
<linux/completion.h>. Fix it.

i2c-powermac includes <linux/completion.h> but doesn't use any
completion. Fix it.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: David Brownell <david-b@pacbell.net>