[PATCH] e1000: avoid leak when e1000_setup_loopback_test fails

In e1000_loopback_test, make sure to call e1000_free_desc_rings if
e1000_setup_loopback_test fails.  Currently in that case it will not
get called, causing a leak.

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