usb: dwc3-msm: remove use of global context variable (ADC)

Remove usage of the global context variable by retrieving the
instance from the dev pointer passed to the sysfs functions. This
will allow the driver to eventually support multiple instances.

Change-Id: If08ddcab8cdb4753c04add1f2b9efa683bcab474
Signed-off-by: Jack Pham <jackp@codeaurora.org>