m68k: Replace memcpy() + manual NUL-termination with strscpy()

Use strscpy() to safely copy the command-line string instead of memcpy()
followed by a manual NUL-termination.

The source string is also NUL-terminated and meets the __must_be_cstr()
requirement of strscpy().

No functional changes intended.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>