staging: dwc2: add const to handling of dwc2_core_params

Now the functions use proper const annotations, the global variable with
default params can be marked const, which prevents these values from
being changed for a specific device (in theory there could be multiple
controllers with different settings, for example).

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
[matthijs@stdin.nl: Split patch from bigger patch, marked
		    dwc2_module_params in pci.c as const and added
		    commit message]
Signed-off-by: Matthijs Kooijman <matthijs@stdin.nl>
Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>