scsi: qedf: Use 'dma_zalloc_coherent' to reduce code verbosity.

Replace some 'dma_alloc_coherent+memset' by some quivalent
'dma_zalloc_coherent' in order to reduce code verbosity

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Chad Dupuis <chad.dupuis@cavium.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>