e1000: rename struct e1000_buffer to e1000_tx_buffer

and remove *page, its only used for Rx.

Signed-off-by: Florian Westphal <fw@strlen.de>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>