USB: serial: increase the number of devices we support

USB: serial: increase the number of devices we support

We had the limit of 255 USB to serial devices on one system for almost
15 years, with no complaints.  But now it's time to move on from these
tiny "baby" systems, and bump the number up to 512, which should last
us a few more years:
	"512 is a nice number" -- Tobias Winter

Note, this is still a static value, and uses up tty core memory with
this many tty devices allocated.  Converting the driver to use
TTY_DRIVER_DYNAMIC_DEV is the next thing to do in order to remove this
limitation.

Reported-by: Tobias Winter <tobias@linuxdingsda.de>
Tested-by: Tobias Winter <tobias@linuxdingsda.de>
Reviewed-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>