[realtek] Fix reopening of legacy-mode 8139 NIC

realtek_destroy_ring() currently does nothing if the card is operating
in legacy (pre-RTL8139C+) mode.  In particular, the producer and
consumer counters are incorrectly left holding their current values.
Virtual hardware (e.g. the emulated RTL8139 in qemu and similar VMs)
is tolerant of this behaviour, but real hardware will fail to transmit
if the descriptors are not used in the correct order.

Fix by resetting the producer and consumer counters in
realtek_destroy_ring() even if the card is operating in legacy mode.

Reported-by: Gelip <mrgelip@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>
Merge branch 'master' into osimages