net: rmnet_usb_ctrl: Avoid double queuing of control buffers

Spurious notification on interrupt endpoint results in the
same get_encapsulated_response URB getting submitted/anchored
multiple times causing list double add bug. Avoid double queuing
of control URB by ignoring the get encapsulated request if control
URB is already in progress.

CRs-Fixed: 619652
Change-Id: I28924dd6b040529c0b5bf544d1185151afb669bb
Signed-off-by: Vamsi Krishna <vskrishn@codeaurora.org>