ols: refactor using max_channels

ols: refactor using max_channels

Let max_channels really carry the number of maximum channels the
hardware supports. We will handle the limitation of only half the
channels available in 200MHz mode later. Note that there won't be a
regression because we only set the variable but never check it. The
desired result of this patch is the removal of the NUM_CHANNELS macro.
The number of channels needs to be dealt with at runtime.

Signed-off-by: Wolfram Sang <redacted>