Staging: et131x: clean up the avail fields in the rx registers

These have a wrap bit but again need little work to clean out. There are a
couple of uglies left that want addressing in later clean up. Notably we should
probably keep the local psr copy and wrap as two values.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>