configure oem_callback to print result on wpa_cli console

wpa_supplicant's callback API for the DRIVER command
over ctrl_iface (wpa_supplicant_driver_cmd) overwrites
the response_buffer if the return value is 0. This will
not allow the response buffer data to be logged on
to wpa_cli console

Hence return the strlen of response_buffer for the
successful case of oem data callback , which gets relayed
to the wpa_cli and accordingly logs the data on console.
For the failure case, return -1 to wpa_supplicant rather
than WPA_DRIVER_OEM_STATUS_FAILURE( which is internal to
wpa_supplicant_lib).

Change-Id: I9b0f88d4908c79aeaeb9686dc2c303e75f75605f
CRs-Fixed: 2732947