ssb: fix interrupt assignment

Explicitely enable shared interrupt 2 for any core that didn't get a dedicated IRQ
anymore (fallthrough case) and for EXTIF cores to make gpio interrupts work.
Also remove a bogus comment.

Signed-off-by: Jochen Friedrich <jochen@scram.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>