scsi: ufs: fix empty check of error history

Currently checking if an error history element is empty or not is by its
"value". In most cases, value is error code.

However this checking is not correct because some errors or events do not
specify any values in error history so values remain as 0, and this will
lead to incorrect empty checking.

Fix it by checking "timestamp" instead of "value" because timestamp will be
always assigned for all history elements

Cc: Alim Akhtar <alim.akhtar@samsung.com>
Cc: Asutosh Das <asutoshd@codeaurora.org>
Cc: Avri Altman <avri.altman@wdc.com>
Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Bean Huo <beanhuo@micron.com>
Cc: Can Guo <cang@codeaurora.org>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Link: https://lore.kernel.org/r/1578147968-30938-2-git-send-email-stanley.chu@mediatek.com
Reviewed-by: Bean Huo <beanhuo@micron.com>
Signed-off-by: Stanley Chu <stanley.chu@mediatek.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>