V4L/DVB (8751): vivi: Fix some issues at vivi register routine

This patch fixes several small issues at vivi register routines:

1) minor and n_devs should be unsigned;
2) n_devs = 0 were not properly handled;
3) if n_devs specify a high number, the driver would just roll back and
won't register any device. The proper behaviour is to keep all succeded
devices registered;
4) both n_devs and minor were using 0 as permissions. Better to have
them with 0444.

With the current patch, if n_devs specify a very large value, it will
register all possible devices. For example, on a machine without any
other V4L drivers loaded, with this patch, we will have something like:

vivi: V4L2 device registered as /dev/video0
vivi: V4L2 device registered as /dev/video1
vivi: V4L2 device registered as /dev/video2
	...
vivi: V4L2 device registered as /dev/video31
video_register_device_index: get_index failed
Video Technology Magazine Virtual Video Capture Board ver 0.5.0 successfully loaded.

5) The number of allocated devices on success is now kept at n_devs:

$ cat /sys/module/vivi/parameters/n_devs
32

Thanks to Henne <henne@nachtwindheim.de> for pointing that there were
some issues at vivi.

Cc: Henne <henne@nachtwindheim.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>