mlxsw: i2c: Fix buffer increment counter for write transaction

It fixes a problem for the last chunk where 'chunk_size' is smaller than
MLXSW_I2C_BLK_MAX and data is copied to the wrong offset, overriding
previous data.

Fixes: 6882b0ae ("mlxsw: Introduce support for I2C bus")
Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>