Improve detection of MAX6642 by reading non existing registers (0x04, 0x06
and 0xff). Reading those registers returns the previously read value.
Signed-off-by: NPer Dalen <per.dalen@appeartv.com>
[guenter.roeck@ericsson.com: added second set of register reads]
Signed-off-by: NGuenter Roeck <guenter.roeck@ericsson.com>