qlcnic: Fix memory corruption while reading stats using ethtool.

o  Driver is doing memset with zero for total number of stats bytes when
   it has already filled some data in the stats buffer, which can overwrite
   memory area beyond the length of stats buffer.

o  Fix this by initializing stats buffer with zero before filling any data in it.

Signed-off-by: Manish Chopra <manish.chopra@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>