[PATCH] i2c-powermac: Fix master_xfer return value

Fix the value returned by the i2c-powermac's master_xfer method.
It should return the number of messages processed successfully, but
instead returns the number of data bytes in the first (and only)
processed message.

Also explicitly mention the master_xfer convention so that future
implementations get it right directly.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>