scsi: ufs: Fix off-by-one bug in ufs debugfs driver

When getting string from userspace by simple_write_to_buffer
in ufs_qcom_dbg_testbus_cfg_write() function, null byte may
be written out of bounds of configuration buffer if return
value is same as size of buffer, causing off-by-one bug.
This change passes correct available size of configuration
buffer to simple_write_to_buffer function.

Change-Id: If6085e806aa34622a9a57b02f563869c51011d2f
Signed-off-by: Sayali Lokhande <sayalil@codeaurora.org>