staging: dwc2: re-use hptxfsiz variable

For some reason, the value of the HPTXFSIZ register was built in the
ptxfsiz variable, while there was also a hptxfsiz variable availble.
Better just use that and remove the (now unused) ptxfsiz variable.

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