usb: dwc3: Get MDWIDTH for DWC_usb32

DWC_usb32 supports MDWIDTH value larger than 255 and up to 1023. The
field HWPARAMS6[9:8] stores the upper 2-bit values of the DWC_usb32's
MDWIDTH. Check that parameter and properly get the MDWIDTH for
DWC_usb32.

Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>