ixgbe: move tc variable to CONFIG_IXGBE_DCB

tc is required by CONFIG_IXGBE_DCB.
This also fixes compilation warning:

 drivers/net/ixgbe/ixgbe_main.c: In function ‘ixgbe_tx_is_paused’:
 drivers/net/ixgbe/ixgbe_main.c:245: warning: unused variable ‘tc’

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Acked-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>