i2c-davinci: behave with i2cdetect

Make i2c-davinci cope properly with "i2cdetect":  don't spew
syslog spam on perfectly normal behaviors, or respond to any
address other than the one reserved for the SMBus host.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>