can: sja1000: fix {pre,post}_irq() handling and IRQ handler return value

commit 2fea6cd303c0d0cd9067da31d873b6a6d5bd75e7 upstream.

This patch fixes the issue that the sja1000_interrupt() function may have
returned IRQ_NONE without processing the optional pre_irq() and post_irq()
function before. Further the irq processing counter 'n' is moved to the end of
the while statement to return correct IRQ_[NONE|HANDLED] values at error
conditions.

Reported-by: Wolfgang Grandegger <wg@grandegger.com>
Acked-by: Wolfgang Grandegger <wg@grandegger.com>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
[bwh: Backported to 3.2: s/SJA1000_IER/REG_IER/; s/SJA1000_IR/REG_IR/]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>