i2c: ismt: Remove call to memset after dmam_alloc_coherent

In commit 518a2f19

 ("dma-mapping: zero memory returned from
dma_alloc_*"), dma_alloc_coherent has already zeroed the memory. So
memset is not needed.

Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Fuqian Huang <huangfq.daxian@gmail.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>