Revert "if_: check for non-empty and >0 before reporting speed (thank…

…s to ssm)"

This reverts commit f982751.

This causes a syntax error when $SPEED is empty; see issue #771 for
example. The original test already did the right thing when $SPEED is
empty: the check for $SPEED being greater than 0 returned false, so
the function continued to the method for determining speed.

plugins/node.d.linux/if_

@@ -90,7 +90,7 @@ findspeed_mbps() {
     # iwlist first)
     if [[ -r /sys/class/net/$INTERFACE/speed ]]; then
             SPEED=$(cat /sys/class/net/$INTERFACE/speed 2>/dev/null)
-            if [ -n "$SPEED" -a "$SPEED" -gt "0" ]; then
+            if [[ "$SPEED" -gt 0 ]]; then
                 echo $SPEED
                 return
             fi