net: tundra: tsi108: use spin_lock_irqsave instead of spin_lock_irq in IRQ context

As spin_unlock_irq will enable interrupts.
Function tsi108_stat_carry is called from interrupt handler tsi108_irq.
Interrupts are enabled in interrupt handler.
Use spin_lock_irqsave/spin_unlock_irqrestore instead of spin_(un)lock_irq
in IRQ context to avoid this.

Signed-off-by: Fuqian Huang <huangfq.daxian@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>