usb: dwc3: use debugfs_print_regs32()

This a use example of the regs32 utilities in debugfs, although
this fuse use ":" as separator between name and value, and debugs
uses "=" (as it looked to me a more common practice).

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>