MUSB: Blackfin: don't fake blackfin_interrupt() result

Commit a5073b52 (musb_gadget: fix unhandled
endpoint 0 IRQs) misses this change to blackfin.c: stop faking successful
result of blackfin_interrupt() and emitting a debug message on an unhandled
interrupt.

Signed-off-by: Sergei Shtylyov <sshtylyov@mvista.com>
Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>