lib: sbi: improve the definition of SBI_IPI_EVENT_MAX

The previous definition had the assumption that the machine word length
is equal to the word length of LONG. Remove this assumption and add a
static check to prevent errors in subsequent modifications.

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