p54pci: increase ring buffer index counter when skipping

I'm afraid, I forgot to add the following lines to
7262d593 ("p54pci: rx tasklet refactoring").

These changes are necessary to ensure loop termination.

Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>