block: skd_main.c: Remove call to memset after dma_alloc_coherent

In commit af7ddd8a
("Merge tag 'dma-mapping-4.21' of git://git.infradead.org/users/hch/dma-mapping"

),
dma_alloc_coherent has already zeroed the memory.
So memset is not needed.

Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Fuqian Huang <huangfq.daxian@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>