qcacld-3.0: Fix read current country code

In populate_dot_11_f_ext_chann_switch_ann API, will read current country
code from mac context which is not have the updated country code.

So, read the current country code directly from regulatory API,
wlan_reg_read_current_country.

Remove unused variables mac->scan.countryCodeCurrent,
mac_ptr->scan.countryCodeDefault and tagCsr11dinfo->countryCode.

Change-Id: Ice64ef1be5b775125ff3bcacfcab7684c6833cad
CRs-Fixed: 3529808