i2c: stub: Add support for SMBus block commands

SMBus block commands are different to I2C block commands since
the returned data is not normally accessible with byte or word
commands on other command offsets. Add linked list of 'block'
commands to support those commands.

Access mechanism is quite simple: Block commands must be written
before they can be read. Subsequent writes can be partial. Block
read commands always return the number of bytes associated with
the longest previous write.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>