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: NYaniv Machani <yanivma@ti.com>
Signed-off-by: NEliad Peller <eliad@wizery.com>
Signed-off-by: NIdo Reis <idor@ti.com>
Signed-off-by: NVictor Goldenshtein <victorg@ti.com>
Signed-off-by: NLuciano Coelho <luciano.coelho@intel.com>