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

Remove usage of the global context variable in the gadget and DBM
functions by passing a dwc3_msm pointer directly.  This will allow
the driver to eventually support multiple instances.

Change-Id: I0fdc5d703b3fd445e83226534f07743461c96f4c
Signed-off-by: Jack Pham <jackp@codeaurora.org>