lib: sbi: Prevent redundant sbi_ipi_process

Multiple harts may try to send IPI to a particular target hart A
in which case the send_ipi() should be called only when the old
value of the hart A ipi_type is zero.

Signed-off-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup@brainfault.org>