kdb: Remove optional size arguments from strscpy() calls

kdb: Remove optional size arguments from strscpy() calls

If the destination buffer has a fixed length, strscpy() automatically
determines the size of the destination buffer using sizeof() if the
argument is omitted. This makes the explicit sizeof() unnecessary.

Furthermore, CMD_BUFLEN is equal to sizeof(kdb_prompt_str) and can also
be removed. Remove them to shorten and simplify the code.

No functional changes intended.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Justin Stitt <justinstitt@google.com>
Link: https://lore.kernel.org/r/20250319163341.2123-2-thorsten.blum@linux.dev
Signed-off-by: Daniel Thompson <daniel@riscstar.com>