can: c_can: c_can_poll(): only read status register after status IRQ

commit 3cb3eaac upstream.

When the status register is read without the status IRQ pending, the
chip may not raise the interrupt line for an upcoming status interrupt
and the driver may miss a status interrupt.

It is critical that the BUSOFF status interrupt is forwarded to the
higher layers, since no more interrupts will follow without
intervention.

Thanks to Wolfgang and Joe for bringing up the first idea.

Signed-off-by: Kurt Van Dijck <dev.kurt@vandijck-laurijssen.be>
Cc: Wolfgang Grandegger <wg@grandegger.com>
Cc: Joe Burmeister <joe.burmeister@devtank.co.uk>
Fixes: fa39b54c ("can: c_can: Get rid of pointless interrupts")
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>