For RTL8139C+ controllers, have controller handle padding short
checksum offload frames. Software workaround used for broken
controllers(RTL8169, RTL8168, RTL8168B) seem to cause watchdog
timeouts on RTL8139C+.
Introduce a new flag RL_FLAG_AUTOPAD to mark automatic padding
feature of controller and set it for RTL8139C+ and controllers that
use new descriptor format. This fixes watchdog timeouts seen on
RTL8139C+.

Reported by:	Dimitri Rodis < DimitriR <> integritasystems dot com >
Tested by:	Dimitri Rodis < DimitriR <> integritasystems dot com >