Adresses below 0x04 and above 0x77 are reserved in the i2c bus
specification, so don't probe these addresses unless requested
explicitly.

Also do more strict boundary checking:

 - ensure start address is greater or equal to zero;
 - don't decrease stopaddr after checking it being greater or equal to
   startaddr.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>