wcn36xx: Fix max channels retrieval

Kernel test robot reported:drivers/net/wireless/ath/wcn36xx/smd.c:943:33:
   sparse: sparse: cast truncates bits from constant value (780 becomes 80)

The 'channels' field is not a simple u8 array but an array of
channel_params. Using sizeof for retrieving the max number of
channels is then wrong.

In practice, it was not an issue, because the sizeof returned
value is 780, which is truncated in min_t (u8) to 80, which is
the value we expect...

Fix that properly using ARRAY_SIZE instead of sizeof.

Fixes: d707f812

 ("wcn36xx: Channel list update before hardware scan")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/1638435732-14657-1-git-send-email-loic.poulain@linaro.org