esoc: mdm-4x: Fix initialization and debug of external mdm

Request for image transfer needs to be send, once request engine
is prepared to handle the request.
Debug of external modem should only be performed if explicitly
requested ie when EXE_DEBUG command is received.

Change-Id: I530f46abffe831c8b607185380bb5422f69d6ccb
Signed-off-by: Hanumant Singh <hanumant@codeaurora.org>