usb: dwc3: Suppress error message about optional clks

Since the glue layer might already be handling the clocks for
the entire block, it is not always required for the core driver
to handle them. Use the _optional variant of devm_clk_bulk_get()
in order to suppress the error about failing to get the clock.

Change-Id: I254f476d2eb81db50c709d8c9f132dc95194ae46
Signed-off-by: Jack Pham <jackp@codeaurora.org>