usb: msm_otg: Fix null ref pointer crash for some targets

Adding a check to ensure bus_scale_table exists in the platform data
before accessing it. This fix is good for targets which do not have
the bus_scale_table defined in the platform data.

Change-Id: I4ec1ce53b4b7cf545e8aae207e611fb8402ab491
Signed-off-by: Ido Shayevitz <idos@codeaurora.org>