[media] DocBook: remove obsolete note from the dbg_g_register doc

<constant>V4L2_CHIP_MATCH_SUBDEV</constant>,

<structfield>match.addr</structfield> selects the nth sub-device.</para>

    <note>

      <title>Success not guaranteed</title>

      <para>Due to a flaw in the Linux &i2c; bus driver these ioctls may

return successfully without actually reading or writing a register. To

catch the most likely failure we recommend a &VIDIOC-DBG-G-CHIP-INFO;

call confirming the presence of the selected &i2c; chip.</para>

    </note>

    <para>These ioctls are optional, not all drivers may support them.

However when a driver supports these ioctls it must also support

&VIDIOC-DBG-G-CHIP-INFO;. Conversely it may support