e1000: ring buffers resources cleanup

Memory leak was found in 2.6.18-rc4 and e1000 7.2.7 from sourceforge: We
should free resources allocated for previous rings if following allocation
fails.

Signed-off-by: Vasily Averin <vvs@sw.ru>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>