USB: f_rmnet: Drop DTR on host suspend

Some Windows hosts are not dropping DTR during suspend, but are
setting DTR as part of resume. Hence, send DTR set to zero to
the Modem when the rmnet interface is suspended.

Change-Id: I9137021e2c4074c71e9142e7dbac2042311b3c4d
Signed-off-by: Chiranjeevi Velempati <cvelempa@codeaurora.org>