qseecom: Fix error handling path in receive_req()

This patch fixes the error handling path in receive_req() function
called from userspace. Issue seen if user is calling the receive_req
function without calling the listener registration. qseecom_find_svc()
should return NULL if there are no services found.

Change-Id: Id51d8f48d7266b5acbc845f7b9ac0068d16b1ede
CRs-fixed: 496927
Signed-off-by: AnilKumar Chimata <anilc@codeaurora.org>