lib: sbi_ipi: Do not ignore errors from sbi_ipi_send()

Currently, failures in sbi_ipi_send() are silently ignored, which makes
them difficult to debug. Instead, abort sending the IPI and pass back
the error, but still synchronize any IPIs already sent.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Signed-off-by: Anup Patel <anup@brainfault.org>