can: sja1000: clear interrupts on start

commit 7cecd9ab upstream.

According to SJA1000 data sheet error-warning (EI) interrupt is not
cleared by setting the controller in to reset-mode.

Then if we have the following case:
- system is suspended (echo mem > /sys/power/state) and SJA1000 is left
  in operating state
- A bus error condition occurs which activates EI interrupt, system is
  still suspended which means EI interrupt will be not be handled nor
  cleared.

If the above two events occur, on resume there is no way to return the
SJA1000 to operating state, except to cycle power to it.

By simply reading the IR register on start we will clear any previous
conditions that could be present.

Signed-off-by: Mirza Krak <mirza.krak@hostmobility.com>
Reported-by: Christian Magnusson <Christian.Magnusson@semcon.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
[lizf: Backported to 3.4: s/SJA1000_IR/REG_IR/]
Signed-off-by: Zefan Li <lizefan@huawei.com>