drivers/rtc/rtc-pcf8563.c: fix pcf8563_irq() error return value

As pointed out by Sergei Shtylyov, the pcf8563_irq function contains a
bug in the error handling: an interrupt handler is not supposed to
return an errno value but an 'enum irqreturn'.

Let's fix this by returning IRQ_NONE in case of a communication error.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>