i2cdetect: only use "newer" I2C_FUNC_* flags if they exist

This allows i2cdetect to be compiled for older kernels.

Reported-by: "Poeche, Uwe" <uwe.poeche@siemens.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Wolfram Sang <wsa@kernel.org>