USB: dwc3: Set gadget max speed as per core's max speed of operation

On custom hardwares of 8974 platform, superspeed mode support is
disabled in hardware by having Highspeed port instead of superspeed
port. WHCK tests might fail if the tests are executed on this platform
due to wrong value of bcdUSB of device descriptor sent to host. This
is due to wrong value set for gadget's max speed. Hence set gadget
maximum speed based on maximum speed at which USB3 core is configured
to operate.

CRs-Fixed: 539890
Change-Id: Ic7d18aa889ebba508c556266dd7f815092b951f6
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>