can: sja1000: fix isr hang when hw is unplugged under load

In the case of hotplug enabled devices (PCMCIA/PCIeC) the removal of the
hardware can cause an infinite loop in the common sja1000 isr.

Use the already retrieved status register to indicate a possible hardware
removal and double check by reading the mode register in sja1000_is_absent.

Cc: stable@kernel.org [3.2+]
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Acked-by: Wolfgang Grandegger <wg@grandegger.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>