Rename qca_wlan_vendor_attr_roam_subcmd to represent subcmds
qca_wlan_vendor_attr_roam_subcmd is an enum associated with the
attribute QCA_WLAN_VENDOR_ATTR_ROAMING_SUBCMD. It represents different
sub command values and these are not the attributes. Hence, rename the
enum to qca_wlan_vendor_roaming_subcmd. Accordingly, the members of this
enum are also renamed to suite the usage.