msm: mdm: Wait for remote mdm to completely shut down

We now wait for the external mdm to drop its status gpio low before
returning from the shutdown ioctl. This is a indication that the mdm
has powered off.

Change-Id: I7bba370c7e9dbac0f9bdcfd6a84f33dcd015d1fb
Signed-off-by: Ameya Thakur <ameyat@codeaurora.org>