fm10k: use actual hardware registers when checking for pending Tx

static u64 fm10k_get_tx_pending(struct fm10k_ring *ring)

{

	/* use SW head and tail until we have real hardware */

	u32 head = ring->next_to_clean;

	u32 tail = ring->next_to_use;

	struct fm10k_intfc *interface = ring->q_vector->interface;

	struct fm10k_hw *hw = &interface->hw;

	u32 head = fm10k_read_reg(hw, FM10K_TDH(ring->reg_idx));

	u32 tail = fm10k_read_reg(hw, FM10K_TDT(ring->reg_idx));

	return ((head <= tail) ? tail : tail + ring->count) - head;

}