e1000: prevent statistics from garbling during bus resets

If a PCI bus error/fault triggers a PCI bus reset, attempts to get
the ethernet packet count statistics from the hardware will fail,
returning garbage data upstream.  This patch skips statistics data
collection if the PCI device is not on the bus.

Signed-off-by: Linas Vepstas <linas@austin.ibm.com>
Cc: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Acked-by: Auke Kok <auke-jan.h.kok@intel.com>