cnss: store the unsafe channel info even channel count is zero

Store WiFi unsafe Channel list and return to wlan host driver on
query. It adds support WiFi unsafe Channel list for LTE-WiFi
coexistence.

When LTE is turned off or some LTE band without any coexistence
issue, The modem can send unsafe channel count as zero to wlan
firmware. The WLAN firmware send the unsafe channel count event
to wlan host driver.

Store the unsafe channel info even the unsafe channel count is zero.

CRs-Fixed: 1018942
Change-Id: I5d2bff8d12d2257387739c1bf8b940aedfdb2269
Signed-off-by: Sarada Prasanna Garnayak <sgarna@codeaurora.org>