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

Remove usage of the global context variable in the charger detection
functions by retrieving the dwc3_msm instance from the dev pointer
passed to the sysfs functions. This will allow the driver to eventually
support multiple instances.

Change-Id: Ia11ec77c04239ae8afacc7dab0b38eaf85024276
Signed-off-by: Jack Pham <jackp@codeaurora.org>