usb: dwc3-msm: rename local variables to mdwc

Do 's/msm/mdwc/' throughout the file as 'msm' is a confusing
name to use as a local variable. This makes usage uniform
and consistent with 'mdwc' used in other functions.

Change-Id: I5f94ec518ee4f0dc1324c7e75ee7c077b13cbe91
Signed-off-by: Jack Pham <jackp@codeaurora.org>