net: stmmac: fix a possible endless loop

It forgot to reduce the value of the variable retry in a while loop
in the ethqos_configure() function. It may cause an endless loop and
without timeout.

Fixes: a7c30e62 ("net: stmmac: Add driver for Qualcomm ethqos")
Signed-off-by: Dejin Zheng <zhengdejin5@gmail.com>
Acked-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>