i2c-nforce2: Add support for SMBus block transactions

Add support for SMBus block read/write transactions to i2c-nforce2
driver, in particular to host controllers MCP51 and MCP55.

Signed-off-by: Oleg Ryjkov <olegr@google.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>