mwifiex: fixes the unexpected be printed log by default

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>

drivers/net/wireless/marvell/mwifiex/cfp.c

@@ -350,7 +350,7 @@ mwifiex_get_cfp(struct mwifiex_private *priv, u8 band, u16 channel, u32 freq)
 		}
 	}
 	if (i == sband->n_channels) {
-		mwifiex_dbg(priv->adapter, ERROR,
+		mwifiex_dbg(priv->adapter, WARN,
 			    "%s: cannot find cfp by band %d\t"
 			    "& channel=%d freq=%d\n",
 			    __func__, band, channel, freq);