e100, e1000, ixgb: Fix an impossible memory overwrite bug

We keep getting requests from people that think that this might be
an exploitable hole where we would overwrite 4 bytes in the netdev
struct if the pci name would exceed 15 characters. In reality this
will never happen but we fix it anyway.

Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>