wlan: Decouple get channel command from extscan ini

Currently there is a check for the gExtscan ini value
within get valid channel command and if gExtscan ini is
disabled, get valid channel command fails.

To address above issue, do not check for the ini value
as the command to get valid channel should be independent
of ini value.

Change-Id: I0cf24b1e96934111c728701b0db0c5f682934acf
CRs-fixed: 2528040