allow LEGACY_PTYS to be set to 0

The count of legacy pty devices can be set by a kernel commandline
parameter. For the distro kernel, we would like to disable all pty's
by default, but keep the opportunity to request devices on the kernel
commandline.

Signed-off-by: Kay Sievers <kay.sievers@vrfy.org>
Acked-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>