scsi: hisi_sas: Limit users changing debugfs BIST count value

scsi: hisi_sas: Limit users changing debugfs BIST count value

BugLink: https://bugs.launchpad.net/bugs/1969107
[ Upstream commit 286ce4c65fbdf5eb9d4d5f4e4997c4e32bf1b073 ]

Add a file operation for "cnt" file under bist directory, so users can only
read "cnt" or clear "cnt" to zero, but cannot randomly modify.

Link: https://lore.kernel.org/r/1645703489-87194-6-git-send-email-john.garry@huawei.com
Signed-off-by: Xiang Chen <chenxiang66@hisilicon.com>
Signed-off-by: Qi Liu <liuqi115@huawei.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
(cherry picked from commit dd111d335cabaddad86075ca5ca3376cd5d1151c)
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>