usb: bam: remove unnecessary pm_runtime call

The disconnect flow for pm_runtime calls is handled
from the gadget. Removing unnecessary - and potentially
buggy - call from the suspend_core function.

Change-Id: I53ff724c5740e5ff0cec525591cb1c77f1778a9c
Signed-off-by: Bar Weiner <bweiner@codeaurora.org>