WifiP2pWfdInfo: Only pass the wfd info to WifiNative

WifiP2pWfdInfo.getDeviceInfoHex() is used to set the WFD info in
wpa_supplicant. The length of this WFD info will be calculated and
prepended by the HIDL interface itself, so there is no need for this
length to prepended in the framework. So, get rid of it.

Bug: 38222615
Test: Compiles
Change-Id: If8ff6e558dad974a053265e0a8c8222943065823