This patch uses WARN level is not printed by default.

In some cases, some boards have always met the unused log be printed as
follows.
...
[23193.523182] mwifiex_pcie 0000:01:00.0: mwifiex_get_cfp:
cannot find cfp by band 2    & channel=13 freq=0
[23378.633684] mwifiex_pcie 0000:01:00.0: mwifiex_get_cfp:
cannot find cfp by band 2    & channel=13 freq=0

Due to we used the wifi default area was US and didn't support 12~14
channels. As Frequencies:
* 2412 MHz [1] (30.0 dBm)
* 2417 MHz [2] (30.0 dBm)
* 2422 MHz [3] (30.0 dBm)
* 2427 MHz [4] (30.0 dBm)
* 2432 MHz [5] (30.0 dBm)
* 2437 MHz [6] (30.0 dBm)
* 2442 MHz [7] (30.0 dBm)
* 2447 MHz [8] (30.0 dBm)
* 2452 MHz [9] (30.0 dBm)
* 2457 MHz [10] (30.0 dBm)
* 2462 MHz [11] (30.0 dBm)
* 2467 MHz [12] (disabled)
* 2472 MHz [13] (disabled)
* 2484 MHz [14] (disabled)

Also, as the commit 1b499cb7
("mwifiex: disable channel filtering feature in firmware"), it proved to
be a feature to get better scan result from overlapping channel.

Even there could be AP from overlapping channel (might be 12/13/14
in this case), it will be filtered depend on reg domain rules.
e.g:
...
if (ch->flags & IEEE80211_CHAN_DISABLED)
        continue;

So it should not been an ERROR, use the WARN level to instead it for now.
Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Acked-by: Xinming Hu <huxm@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>