v4l2: fix probing and enumeration of stepwise frame sizes

v4l2: fix probing and enumeration of stepwise frame sizes

The code enumerating STEPWISE framesizes would start from
(min_w, min_h) and then add (step_w, step_h) to get the
next framesize. However, it should really allow any width
from min_w to max_w with step_w and same for heights.
Secondly, we would add and probe each individual stepped
frame size to the caps as separate structure, which would
lead to hundreds if not thousands of structs ending up in
the probed caps. Use integer ranges with steps instead.

This was particularly noticable with the Raspberry Pi Cam.

https://bugzilla.gnome.org/show_bug.cgi?id=724521
https://bugzilla.gnome.org/show_bug.cgi?id=732458
https://bugzilla.gnome.org/show_bug.cgi?id=726521