staging: dwc2: only read the snpsid register once

This (read-only) register was read twice, storing it for later use the
second time. Now it is only read once, storing it right away.

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