staging: dwc2: add missing shift

This line extracted the available queue space without properly shifting
it. Since the code only cared wether it was zero or not, it worked as
expected without the shift, but adding shift makes the code cleaner.

While we're here, store the result in a helper variable that was already
declared to increase readability a bit more.

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