scsi: ufs-debugfs: overwrite error scenario value via debugfs

In the old code, there is no way to "disable" the already enabled
error scenarios since the operator in use is a bit-wise OR operator.
This change fixes this issue. Now, the new value overwrites the previous
value so it's possible to disable all error scenarios by:
echo 0 > /d/ufshcd0/err_inj_scenario

Change-Id: I21eadb3345f6587edf5232ea3c03702bfdb930ad
Signed-off-by: Yaniv Gardi <ygardi@codeaurora.org>