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>