SJA1000 driver interrupts processing modified to not rely on interrupt register for...

The information about free chip ready condition for Tx message is now
obtained from status register. This solution enables us to equip SJA1000
driver by irq_accept method. Then only this part has to be run in real IRQ
handler and rest of ISR can be done in other thread. This is required
for possible porting to QNX as well. Change should helps to better
recover form bus-off mode as well.