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

Remove usage of the global context variable in the regulator and
block reset helper functions by passing the dwc3_msm pointer as a
parameter. This will allow the driver to eventually support multiple
instances.

Change-Id: I4e6892ff71a1482c319b6633e7a1ed128c93a3bf
Signed-off-by: Jack Pham <jackp@codeaurora.org>