drivers/char/vt_ioctl.c: fix VT_OPENQRY error value

When all VT's are in use, VT_OPENQRY casts -1 to unsigned char before
returning it to userspace as an int.  VT255 is not the next available
console.

Signed-off-by: Graham Gower <graham.gower@gmail.com>
Cc: Greg KH <greg@kroah.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>