net: ethernet: slicoss: remove redundant initialization of idx

Variable idx is being initialized and later on over-written by
a new value in a do-loop without the initial value ever being
read. Hence the initializion is redundant and can be removed.
Cleans up clang warning:

drivers/net/ethernet/alacritech/slicoss.c:358:15: warning: Value
stored to 'idx' during its initialization is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>