usb: msm_otg: allow debugfs control of BSV in peripheral mode

Allow msm_otg device to be configured in peripheral mode
simultaneously with debugfs control of OTG mode. This allows
the user to manually switch on/off the BSV bit via debugfs.

Example with device tree:
  qcom,hsusb-otg-mode = <1>;    // peripheral mode
  qcom,hsusb-otg-control = <3>; // user control

To turn on BSV (already on by default):
> echo "peripheral" > <debugfs>/msm_otg/mode

To turn off BSV:
> echo "none" > <debugfs>/msm_otg/mode

Change-Id: Ida2e7e5601d387b36d57da0643f3ed17f375742b
Signed-off-by: Jack Pham <jackp@codeaurora.org>