wlan: Accept cfg80211_set_spoofed_mac_oui only in STA mode

Presently there is no check to see in which adapter mode the command is
being issued. This can cause the further layers to assume STA interface
to be present even though it is not, leading to an assert.

Add the check to ensure that this command is processed only in the case
of STA mode.

Change-Id: I6b264b62fd56525e16761cc43683e52fbbe9ccfc
CRs-Fixed: 2341551