Skip unnecessary processing in interrupt handler.
Skip unnecessary processing in interrupt handler.

2 spc(4/luna88k)'s share the same level 3 interrupt on LUNA-88K2.  So
returns earlier when the device is not the source of interrupt.

NetBSD has the same code.
http://cvsweb.netbsd.org/bsdweb.cgi/src/sys/dev/ic/mb89352.c#rev1.34

"it is the right thing to do" miod@

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars