riscv: Fix secondary_entry

The last few instructions of secondary_entry had the right concept,
but were the totally wrong implementation. Without setting ra, then,
when the boot function doesn't stay in an infinite loop, like
do_idle() would, we'd go off into the weeds when trying to return
from it. Make sure we set ra to come back to where we can then call
do_idle() instead. The bug was found by inspection since nobody is
calling smp_boot_secondary() with anything other than do_idle() at
this time.

Fixes: 9c92b28e ("riscv: Add SMP support")
Signed-off-by: Andrew Jones <andrew.jones@linux.dev>