net: stmmac: xgmac: Fix TSA selection

When we change between Transmission Scheduling Algorithms, we need to
clear previous values so that the new chosen algorithm is correctly
selected.

Fixes: ec6ea8e3 ("net: stmmac: Add CBS support in XGMAC2")
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>