#184 Avoid negative numbers in PTZ presets

[flreinhard]

Fixes #184

[roleoroleo]

I don't understand exactly this pr.
Coordinates cannot be negative, at least in my model.

[flreinhard]

On my cam that's easy to reproduce:

1. Move the camera almost to the right limit
2. use ptz -a right
3. you'll notice/hear, that after reaching the right limit, the motor will still try to push it further but can't
4. result is a negative x coordinate
5. once you've had a negative x, all preset's won't work any more, since you'll need to do a homing cycle

[root@TestCam]# ptz -a get_coord
Current x  = 201, y  = 450
Degrees x  = 339.3, y  = 60.0
339.3,60.0
[root@TestCam]# ptz -a right 
[root@TestCam]# ptz -a get_coord
Current x  = -1919, y  = 450
Degrees x  = 557.4, y  = 60.0
557.4,60.0
[root@TestCam]# cat /mnt/mmc/sonoff-hack/model 
GK-200MP2-B

is it possible to trigger homing manually?