u_qc_ether: Decrement rndis network device count

Currently rndis0 network device count is being incremented in
rndis_qc_set_alt() when taking reference to netdev related to
rndis0 by using gether_qc_get_net() API. As this reference count
is not being decremented, rndis0 netdevice is not being released.
This fails composition change while changing from RNDIS based one
to default composition. Fix this refcount mismatch by decrementing
the same in gether_qc_get_net() API.

CRs-Fixed: 611543
Change-Id: I74fe5bbe6d81ab3e9a3703711ebd4d4afc8310b7
Signed-off-by: Mayank Rana <mrana@codeaurora.org>