net: usb: Prevent suspend if response available notification received

There is a possibility of interface suspend after receiving response
available notification from mdm device. In this case driver unlinks
the rx urb and do not send GET_ENCAPSULATED_RESPONSE setup packet to
mdm. Response to mdm gets delayed up until interface resumes and mdm
sends response available notification again. Hence resume interface
after receiving response available notification if it is suspended.
Allow interface suspend only after completing
GET_ENCAPSULATED_RESPONSE.

Change-Id: Ib52e64b1cc7b6d5edfe3ada9651ec56526cf4852
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>