nios2: fix timer initcall return value

When called more than twice, the nios2_time_init() function return an
uninitialized value, as detected by gcc -Wmaybe-uninitialized

  arch/nios2/kernel/time.c: warning: 'ret' may be used uninitialized in this function

This makes it return '0' here, matching the comment above the function.

Acked-by: Ley Foon Tan <lftan@altera.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>