i2c: xgene: Avoid dma_buffer overrun

SMBus block command uses the first byte of buffer for the data length.
The dma_buffer should be increased by 1 to avoid the overrun issue.

Reported-by: Phil Endecott <phil_gjouf_endecott@chezphil.org>
Signed-off-by: Hoan Tran <hotran@apm.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Cc: stable@kernel.org