regmap: i2c: fallback to SMBus if the adapter does not support standard I2C

Some I2C adapters are only compatible with the SMBus protocol and do not
support standard I2C transfers.

Fallback to SMBus transfers if we encounter such kind of adapters.
The transfer type is chosen according to the val_bits field in the regmap
config.

Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Signed-off-by: Mark Brown <broonie@linaro.org>