ixgbe: drop unexpected word 'for' in comments

there is an unexpected word 'for' in the comments that need to be dropped

file - drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
line - 5164

 * ixgbe_lpbthresh - calculate low water mark for for flow control

changed to:

 * ixgbe_lpbthresh - calculate low water mark for flow control
Signed-off-by: Jiang Jian <jiangjian@cdjrlc.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/ixgbe/ixgbe_main.c

@@ -5161,7 +5161,7 @@ static int ixgbe_hpbthresh(struct ixgbe_adapter *adapter, int pb)
 }
 
 /**
- * ixgbe_lpbthresh - calculate low water mark for for flow control
+ * ixgbe_lpbthresh - calculate low water mark for flow control
  *
  * @adapter: board private structure to calculate for
  * @pb: packet buffer to calculate