usb: dwc3: Turn on GDSC before enabling cfg_ahb_clk

On some targets cfg_ahb_clk depends upon usb gdsc regulator.
If cfg_ahb_clk is enabled without turning on usb gdsc regulator
clk is stuck off. This causes un-clocked register access.

Change-Id: I3e45b1b27a652a4d9c167ea6edbe9df8c2abee30
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>