wpa_supplicant(hidl): Prepend length for WFD IE

|wifi_display_subelem_set| expects the first 2 bytes of each IE being
set to contain the length of that IE. So, prepend that in the info
string being constructed in the HIDL interface.

Bug: 38222615
Test: Compiles
Change-Id: I190b9b5ad27d9fda17393302c697a8ea1c23df6c
1 file changed