scsi: ufs: fix debugfs tag stats resetting

When attempting to clear ufs tag stats, memset 0 is called with size
of multiples of unsigned int.
This is wrong since each cell in the stats array is of type (u64).
Fix the memset to sizeof(*tag_stats).

Change-Id: Iaeb70616a2160ee9cba4605e7251dc45868d1951
Signed-off-by: Lee Susman <lsusman@codeaurora.org>