accel/ivpu: refactor deprecated strncpy

`strncpy` is deprecated for use on NUL-terminated destination strings [1].

A suitable replacement is `strscpy` [2] due to the fact that it
guarantees NUL-termination on its destination buffer argument which is
_not_ the case for `strncpy`!

Also remove extraneous if-statement as it can never be entered. The
return value from `strncpy` is it's first argument. In this case,
`...dyndbg_cmd` is an array:
| 	char dyndbg_cmd[VPU_DYNDBG_CMD_MAX_LEN];
             ^^^^^^^^^^
This can never be NULL which means `strncpy`'s return value cannot be
NULL here. Just use `strscpy` which is more robust and results in
simpler and less ambiguous code.

Moreover, remove needless `... - 1` as `strscpy`'s implementation
ensures NUL-termination and we do not need to carefully dance around
ending boundaries with a "- 1" anymore.

Fixes: 5d7422cf ("accel/ivpu: Add IPC driver and JSM messages")
Link: www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
Link: https://github.com/KSPP/linux/issues/90


Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Stanislaw Gruszka <stanislaw.gruszka@linux.intel.com>
Signed-off-by: Stanislaw Gruszka <stanislaw.gruszka@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230824-strncpy-drivers-accel-ivpu-ivpu_jsm_msg-c-v1-1-12d9b52d2dff@google.com