compat_ioctl: Supress "unknown cmd" message on serial /dev/console

After the commit fb07a5f8

 ("compat_ioctl: remove all VT ioctl
handling"), I got this error message on 64-bit mips kernel with 32-bit
busybox userland:

ioctl32(init:1): Unknown cmd fd(0) cmd(00005600){t:'V';sz:0} arg(7fd76480) on /dev/console

The cmd 5600 is VT_OPENQRY.  The busybox's init issues this ioctl to
know vt-console or serial-console.  If the console was serial console,
VT ioctls are not handled by the serial driver.

And by quick search, I found some programs using VT_GETMODE to check
vt-console is available or not.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>