MIPS: Avoid buffer overrun in mips_cm_error_report

Commit 3885c2b4 ("MIPS: CM: Add support for reporting CM cache
errors") added cases for decoding errors reported by CM3, but leaves the
buf variable which is printed as a string uninitialised for cause values
other than 1, 2 or 3. Fix by ensuring the buf variable is initialised to
an empty string in such cases.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/11187/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>