scsi: isci: Make scu_link_layer_set_txcomsas_timeout() static

This addresses the following sparse warning:

drivers/scsi/isci/phy.c:672:6: warning: symbol
'scu_link_layer_set_txcomsas_timeout' was not declared. Should it be
static?

Link: https://lore.kernel.org/r/20200915084000.2826741-1-yanaijie@huawei.com
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>