This is qcacld-2.0 to qcacld-3.0 propagation
Currently min time for dwell time for ext scan is non-zero while it
is zero for all other type of scan. This patch makes min value for
dwell time for ext scan as zero and consistent with other scans.
Change-Id: I7b6f3681c56918af7cdc381d25d6b625a5bdbda3
CRs-Fixed: 923458