scsi: ufs: Fix stack overflow read in ufs debugfs driver

When getting string from userspace by simple_write_to_buffer
in ufs_qcom_dbg_testbus_cfg_write() function, the string
copied to configuration is not terminated with '\0'. Thus
stack overflow read may occur while copying configuration to
host->testbus.select_major, which will result in information
leak later while printing error message. This change adds null
character at the end of the input string to avoid information
leak.

Change-Id: Ic9a9204def4bd6976f42f5f80ae5c0a9730afeb1
Signed-off-by: Sayali Lokhande <sayalil@codeaurora.org>