Driver "init" function might be called multiple times.
On every "init" Tx/Rx buffer descriptors are initialized: "descs_init"
-> "{tx|rx}_descs_init".

In its turn those init functions set MAC's "{tx|rx}desclistaddr" to
point on the first buffer descriptor in the list.

So CPU to start operation from the first buffer descriptor as well after
every "init" we have to reset "{tx|rx}_currdescnum".

[Original U-Boot patch by Alexey Brodkin <abrodkin@synopsys.com>]

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>