[PATCH] I2C: pcf8574 driver cleanup

I found a possible cleanup in the pcf8574 driver. We don't need to store
the read value in our private data structure, as we then never use it
again. I asked Aurelien and he is fine with the change. Please apply,
thanks.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>