Channels 52-64 were mapped incorrectly.

Refactor and document wlcore_get_reg_conf_ch_idx() in
order to make it clear what's going on there.

While on it, fix the return value check to consider
0 as a valid return value as well (indicates channel 1).
Reported-by: Yaniv Machani <yanivma@ti.com>
Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Ido Reis <idor@ti.com>
Signed-off-by: Victor Goldenshtein <victorg@ti.com>
Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>