scsi: isci: Remove unnecessary struct declaration

struct sci_phy_proto was already defined on line 142. The declaration here
is unnecessary. Remove it.

Link: https://lore.kernel.org/r/20210406105913.676746-1-wanjiabing@vivo.com


Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>