staging: dwc2: make dwc2_core_params documentation more complete

Some of the defaults were missing or unclear. In particular, I suspect
the defaults were documented assuming there were still module parameters
and taking the default module parameters into account. Now, the defaults
are the values that will get chosen when the params passed to
dwc2_hcd_init are all -1.

Signed-off-by: Matthijs Kooijman <matthijs@stdin.nl>
Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>