icnss_utils: replace mutex lock by spinlock

Replace mutex lock by spinlock in icnss_wlan_set_dfs_nol
as dfs module may call this function in timer context
which will result apps crash.

CRs-Fixed: 2044689
Change-Id: Ib820589cd8b120a39536aab69d6954ba8cc35e3c
Signed-off-by: Arif Hussain <arifhussain@codeaurora.org>
Signed-off-by: Yuanyuan Liu <yuanliu@codeaurora.org>