f_qc_rndis: Check config or cdev is NULL in before accessing

RNDIS control path completion handlers are getting called during
disconnect as part of composition switch and this is leading to a
crash. Avoid this crash, by checking, if cdev is not NULL before
accessing.

CRs-Fixed: 717035
Change-Id: Id8748f963298129a403ffd6e4413476013315061
Signed-off-by: Mayank Rana <mrana@codeaurora.org>