lib: sbi: sse: call enable callback before sending IPI

Move the enable callback call before sending the IPI. Even though the
event is locked and no race condition can happen, this is more logical.

Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reported-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>