net/designware: add explicit reset of {tx|rx}_currdescnum

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".

Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Vipin Kumar <vipin.kumar@st.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Mischa Jonker <mjonker@synopsys.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>