scsi: ibmvscsi: Remove unnecessary cast

Fix the following coccicheck warning:
WARNING: casting value returned by memory allocation function is useless.

Link: https://lore.kernel.org/r/1615515534-1250-1-git-send-email-wangqing@vivo.com


Signed-off-by: Wang Qing <wangqing@vivo.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>