usb: dwc3: debugfs: fix regdump offset

As with dwc_readl/writel, the global registers are specified as
offsets starting from the beginning of the xHCI address space,
but the memory region pointed to by dwc->regs already maps to
the start of the global addresses. Fix by offsetting each of the
regs relative to DWC3_GLOBALS_REGS_START.

Signed-off-by: Jack Pham <jackp@codeaurora.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>