arch/frv/kernel/setup.c: use strncmp() instead of memcmp()

'cmdline' is a NUL terminated string, when its length < 4, memcmp()
will cause memory access out of boundary.

So use strncmp() instead of memcmp().

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>