USB: rmnet: Handle case when response queue is empty

When there is no packet available in response queue on device side
and host requests for GET_ENCAPSULATED response on control endpoint,
frmnet_setup() API returns as -EOPNOTSUPP. This results into controller
driver stalling EP0 OUT endpoint which host side driver may not
expected. Hence to avoid this stalling of EP0 OUT endpoint returns
zero instead of -EOPNOTSUPP from frment_setup() API.

Change-Id: I82fb30a7113ad61212d47835318701badadb289a
CRs-Fixed: 782966
Signed-off-by: Mayank Rana <mrana@codeaurora.org>