gspca-gl860 driver update

Hello

Here is three patches for the gspca_gl860 driver.
The main patch is the second one, it is a rewrite for the sensor MI2020.

These patches have been proposed on February 28th and refused some days later because of a concern about the use of "udelay" instead of "msleep".
Compared to February, there is no more use of "udelay". I also mapped a new setting to an already existing one in V4L2 instead of the new one.