[PATCH] e1000: zero-out pointers in e1000_free_desc_rings

In e1000_free_desc_rings, zero-out pointers after the memory they
point to is freed.  The test rings are static and get re-used, and
failures during subsequent test setups can cause e1000_free_desc_rings
to get called with dirty pointers.  Dirty pointers can cause oopses
or crashes...

Signed-off-by: John W. Linville <linville@tuxdriver.com>